diff --git a/app/models/queries/custom_fields/hierarchy/item_query.rb b/app/models/queries/custom_fields/hierarchy/item_query.rb
new file mode 100644
index 000000000000..a1bd47a6eceb
--- /dev/null
+++ b/app/models/queries/custom_fields/hierarchy/item_query.rb
@@ -0,0 +1,42 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module Queries
+  module CustomFields
+    module Hierarchy
+      class ItemQuery
+        include ::Queries::BaseQuery
+        include ::Queries::UnpersistedQuery
+
+        def self.model
+          CustomField::Hierarchy::Item
+        end
+      end
+    end
+  end
+end
diff --git a/app/services/custom_fields/hierarchy/hierarchical_item_service.rb b/app/services/custom_fields/hierarchy/hierarchical_item_service.rb
index e0274491d148..49b5f882da13 100644
--- a/app/services/custom_fields/hierarchy/hierarchical_item_service.rb
+++ b/app/services/custom_fields/hierarchy/hierarchical_item_service.rb
@@ -112,11 +112,23 @@ def reorder_item(item:, new_sort_order:)
         Success()
       end
 
-      def soft_delete_item(item)
+      def soft_delete_item(item:)
         # Soft delete the item and children
         raise NotImplementedError
       end
 
+      def hashed_subtree(item:, depth:)
+        if depth >= 0
+          Success(item.hash_tree(limit_depth: depth + 1))
+        else
+          Success(item.hash_tree)
+        end
+      end
+
+      def descendant_of?(item:, parent:)
+        item.descendant_of?(parent) ? Success() : Failure()
+      end
+
       private
 
       def create_root_item(custom_field)
diff --git a/config/locales/en.yml b/config/locales/en.yml
index c47247097270..04cb1b4cbb6e 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -295,19 +295,25 @@ en:
 
   dry_validation:
     errors:
+      integer?: "must be an integer"
+      filled?: "must be filled"
+      greater_or_equal_zero: "must be greater or equal to 0"
+      not_found: "not found"
       rules:
+        item:
+          root_item: "cannot be a root item"
+          not_persisted: "must be an already existing item"
         label:
-          filled?: "must be filled"
           not_unique: "must be unique within the same hierarchy level"
         short:
           not_unique: "must be unique within the same hierarchy level"
-        item:
-          root_item: "cannot be a root item"
-          not_persisted: "must be an already existing item"
+        parent:
+          not_descendant: "must be a descendant of the hierarchy root"
     rules:
+      depth: "Depth"
+      item: "Item"
       label: "Label"
       short: "Short"
-      item: "Item"
       parent: "Parent"
 
   global_search:
@@ -1599,7 +1605,7 @@ en:
   create_new_page: "Wiki page"
 
   date:
-    abbr_day_names: ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]
+    abbr_day_names: [ "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" ]
     abbr_month_names:
       [
         ~,
diff --git a/docs/api/apiv3/components/examples/hierarchy_item_collection_filtered_response.yml b/docs/api/apiv3/components/examples/hierarchy_item_collection_filtered_response.yml
new file mode 100644
index 000000000000..f56cebe3018d
--- /dev/null
+++ b/docs/api/apiv3/components/examples/hierarchy_item_collection_filtered_response.yml
@@ -0,0 +1,62 @@
+description: |-
+  Filtered response of a hierarchy structure starting at a specific parent and restricting the depth.
+value:
+  _type: Collection
+  total: 3
+  count: 3
+  _embedded:
+    elements:
+      - _type: HierarchyItem
+        id: 1338
+        label: Stormtroopers
+        short: ST
+        depth: 1
+        _links:
+          self:
+            href: /api/v3/custom_field_items/1338
+            name: Stormtroopers
+          parent:
+            href: /api/v3/custom_field_items/1337
+            name: null
+          children:
+            - href: /api/v3/custom_field_items/1340
+              name: ST-377
+            - href: /api/v3/custom_field_items/1341
+              name: ST-422
+      - _type: HierarchyItem
+        id: 1340
+        label: ST-377
+        short: null
+        depth: 2
+        _links:
+          self:
+            href: /api/v3/custom_field_items/1340
+            name: ST-377
+          parent:
+            href: /api/v3/custom_field_items/1338
+            name: Stormtroopers
+          children:
+            - href: /api/v3/custom_field_items/1480
+              name: ST-377-200
+            - href: /api/v3/custom_field_items/1481
+              name: ST-377-201
+      - _type: HierarchyItem
+        id: 1341
+        label: ST-422
+        short: null
+        depth: 2
+        _links:
+          self:
+            href: /api/v3/custom_field_items/1341
+            name: ST-422
+          parent:
+            href: /api/v3/custom_field_items/1338
+            name: Stormtroopers
+          children:
+            - href: /api/v3/custom_field_items/1580
+              name: ST-422-137
+            - href: /api/v3/custom_field_items/1581
+              name: ST-422-138
+  _links:
+    self:
+      href: /api/v3/custom_field/42/items?parent=1338&depth=1
diff --git a/docs/api/apiv3/components/examples/hierarchy_item_collection_response.yml b/docs/api/apiv3/components/examples/hierarchy_item_collection_response.yml
new file mode 100644
index 000000000000..0fd692dd746e
--- /dev/null
+++ b/docs/api/apiv3/components/examples/hierarchy_item_collection_response.yml
@@ -0,0 +1,51 @@
+description: |-
+  Simple response of a hierarchy structure starting at root.
+value:
+  _type: Collection
+  total: 37
+  count: 37
+  _embedded:
+    elements:
+      - _type: HierarchyItem
+        id: 1337
+        label: null
+        short: null
+        depth: 0
+        _links:
+          self:
+            href: /api/v3/custom_field_items/1337
+            name: null
+          parent:
+            href: null
+          children:
+            - href: /api/v3/custom_field_items/1338
+              name: Stormtroopers
+            - href: /api/v3/custom_field_items/1339
+              name: Dark Troopers
+      - _type: HierarchyItem
+        id: 1338
+        label: Stormtroopers
+        short: ST
+        depth: 1
+        _links:
+          self:
+            href: /api/v3/custom_field_items/1338
+            name: Stormtroopers
+          parent:
+            href: /api/v3/custom_field_items/1337
+            name: null
+          children:
+            - href: /api/v3/custom_field_items/1340
+              name: ST-377
+            - href: /api/v3/custom_field_items/1341
+              name: ST-422
+      - _type: HierarchyItem
+        _hint: hierarchy item shortened for brevity
+        id: 1340
+        label: ST-377
+        short: null
+        depth: 2
+      - _hint: hierarchy item shortened for brevity
+  _links:
+    self:
+      href: /api/v3/custom_field/42/items
diff --git a/docs/api/apiv3/components/examples/hierarchy_item_response.yml b/docs/api/apiv3/components/examples/hierarchy_item_response.yml
new file mode 100644
index 000000000000..17568d048ca7
--- /dev/null
+++ b/docs/api/apiv3/components/examples/hierarchy_item_response.yml
@@ -0,0 +1,20 @@
+description: |-
+  Simple response of a single item of a custom field of type hierarchy.
+value:
+  _type: HierarchyItem
+  id: 1338
+  label: Stormtroopers
+  short: ST
+  depth: 1
+  _links:
+    self:
+      href: /api/v3/custom_field_items/1338
+      name: Stormtroopers
+    parent:
+      href: /api/v3/custom_field_items/1337
+      name: null
+    children:
+      - href: /api/v3/custom_field_items/1340
+        name: ST-377
+      - href: /api/v3/custom_field_items/1341
+        name: ST-422
diff --git a/docs/api/apiv3/components/schemas/hierarchy_item_collection_model.yml b/docs/api/apiv3/components/schemas/hierarchy_item_collection_model.yml
new file mode 100644
index 000000000000..c6b2a5894737
--- /dev/null
+++ b/docs/api/apiv3/components/schemas/hierarchy_item_collection_model.yml
@@ -0,0 +1,30 @@
+# Schema: HierarchyItemCollectionModel
+---
+allOf:
+  - $ref: './collection_model.yml'
+  - type: object
+    required:
+      - _links
+      - _embedded
+    properties:
+      _links:
+        type: object
+        required:
+          - self
+        properties:
+          self:
+            allOf:
+              - $ref: './link.yml'
+              - description: |-
+                  This hierarchy item collection
+                  
+                  **Resource**: HierarchyItemCollectionModel
+      _embedded:
+        type: object
+        required:
+          - elements
+        properties:
+          elements:
+            type: array
+            items:
+              $ref: './hierarchy_item_read_model.yml'
diff --git a/docs/api/apiv3/components/schemas/hierarchy_item_read_model.yml b/docs/api/apiv3/components/schemas/hierarchy_item_read_model.yml
new file mode 100644
index 000000000000..6b427cddf872
--- /dev/null
+++ b/docs/api/apiv3/components/schemas/hierarchy_item_read_model.yml
@@ -0,0 +1,46 @@
+# Schema: HierarchyItemReadModel
+---
+type: object
+properties:
+  _type:
+    type: string
+    enum:
+      - HierarchyItem
+  id:
+    type: integer
+    description: Hierarchy item identifier
+  label:
+    type: string
+    description: The label of the hierarchy item
+  short:
+    type: string
+    description: The short name of the hierarchy item
+  depth:
+    type: integer
+    description: The hierarchy depth. The root item has a depth of 0.
+  _links:
+    type: object
+    properties:
+      self:
+        allOf:
+          - $ref: './link.yml'
+          - description: |-
+              This hierarchy item
+
+              **Resource**: HierarchyItem
+      parent:
+        allOf:
+          - $ref: './link.yml'
+          - description: |-
+              The hierarchy item that is the parent of the current hierarchy item
+
+              **Resource**: HierarchyItem
+      children:
+        type: array
+        items:
+          allOf:
+            - $ref: './link.yml'
+            - description: |-
+                A hierarchy item that is a child of the current hierarchy item
+                
+                **Resource**: HierarchyItem
diff --git a/docs/api/apiv3/openapi-spec.yml b/docs/api/apiv3/openapi-spec.yml
index 9175c3f08b8b..8b08defb7108 100644
--- a/docs/api/apiv3/openapi-spec.yml
+++ b/docs/api/apiv3/openapi-spec.yml
@@ -186,6 +186,10 @@ paths:
     "$ref": "./paths/custom_action.yml"
   "/api/v3/custom_actions/{id}/execute":
     "$ref": "./paths/custom_action_execute.yml"
+  "/api/v3/custom_fields/{id}/items":
+    "$ref": "./paths/custom_field_items.yml"
+  "/api/v3/custom_field_items/{id}":
+    "$ref": "./paths/custom_field_item.yml"
   "/api/v3/custom_options/{id}":
     "$ref": "./paths/custom_option.yml"
   "/api/v3/days/non_working":
@@ -489,6 +493,12 @@ components:
       $ref: "./components/examples/grid-simple-response.yml"
     GroupResponse:
       $ref: "./components/examples/group-response.yml"
+    HierarchyItemCollectionFilteredResponse:
+      $ref: "./components/examples/hierarchy_item_collection_filtered_response.yml"
+    HierarchyItemCollectionResponse:
+      $ref: "./components/examples/hierarchy_item_collection_response.yml"
+    HierarchyItemResponse:
+      $ref: "./components/examples/hierarchy_item_response.yml"
     MembershipCreateRequestCustomMessage:
       $ref: "./components/examples/membership-create-request-custom-message.yml"
     MembershipCreateRequestGlobalRole:
@@ -673,6 +683,10 @@ components:
       "$ref": "./components/schemas/help_text_collection_model.yml"
     HelpTextModel:
       "$ref": "./components/schemas/help_text_model.yml"
+    HierarchyItemCollectionModel:
+      "$ref": "./components/schemas/hierarchy_item_collection_model.yml"
+    HierarchyItemReadModel:
+      "$ref": "./components/schemas/hierarchy_item_read_model.yml"
     Link:
       "$ref": "./components/schemas/link.yml"
     List_actionsModel:
diff --git a/docs/api/apiv3/paths/custom_field_item.yml b/docs/api/apiv3/paths/custom_field_item.yml
new file mode 100644
index 000000000000..c1ac90dda38b
--- /dev/null
+++ b/docs/api/apiv3/paths/custom_field_item.yml
@@ -0,0 +1,49 @@
+# /api/v3/custom_field_items/{id}
+---
+get:
+  summary: Get a custom field hierarchy item
+  operationId: get_custom_field_item
+  description: |-
+    Retrieves a single custom field item specified by its unique identifier.
+  parameters:
+    - name: id
+      description: The custom field item's unique identifier
+      in: path
+      example: '42'
+      required: true
+      schema:
+        type: integer
+  responses:
+    '200':
+      description: OK
+      content:
+        application/hal+json:
+          schema:
+            $ref: '../components/schemas/hierarchy_item_read_model.yml'
+          examples:
+            'simple response':
+              $ref: '../components/examples/hierarchy_item_response.yml'
+    '403':
+      description: Returned if the user is not logged in.
+      content:
+        application/hal+json:
+          schema:
+            $ref: '../components/schemas/error_response.yml'
+          examples:
+            response:
+              value:
+                _type: Error
+                errorIdentifier: urn:openproject-org:api:v3:errors:MissingPermission
+                message: You are not authorized to access this resource.
+    '404':
+      description: Returned if the custom field item does not exist or the user is not logged in.
+      content:
+        application/hal+json:
+          schema:
+            $ref: '../components/schemas/error_response.yml'
+          examples:
+            response:
+              value:
+                _type: Error
+                errorIdentifier: urn:openproject-org:api:v3:errors:NotFound
+                message: The requested resource could not be found.
diff --git a/docs/api/apiv3/paths/custom_field_items.yml b/docs/api/apiv3/paths/custom_field_items.yml
new file mode 100644
index 000000000000..e51e9e3bffe2
--- /dev/null
+++ b/docs/api/apiv3/paths/custom_field_items.yml
@@ -0,0 +1,87 @@
+# /api/v3/custom_field/{id}/items
+---
+get:
+  summary: Get the custom field hierarchy items
+  operationId: get_custom_field_items
+  description: |-
+    Retrieves the hierarchy of custom fields.
+
+    The hierarchy is a tree structure of hierarchy items. It is represented as a flat list of items, where each item
+    has a reference to its parent and children. The list is ordered in a depth-first manner. The first item is the
+    requested parent. If parent was unset, the root item is returned as first element.
+    
+    Passing the `depth` query parameter allows to limit the depth of the hierarchy. If the depth is unset, the full
+    hierarchy tree is returned. If the depth is set to `0`, only the requested parent is returned. Any other positive
+    integer will return the number of children levels specified by this value.
+    
+    This endpoint only returns, if the custom field is of type `hierarchy`.
+  parameters:
+    - name: id
+      description: The custom field's unique identifier
+      in: path
+      example: '42'
+      required: true
+      schema:
+        type: integer
+    - name: parent
+      description: The identifier of the parent hierarchy item
+      in: query
+      example: '1337'
+      required: false
+      schema:
+        type: integer
+    - name: depth
+      description: The level of hierarchy depth
+      in: query
+      example: '1'
+      required: false
+      schema:
+        type: integer
+  responses:
+    '200':
+      description: OK
+      content:
+        application/hal+json:
+          schema:
+            $ref: '../components/schemas/hierarchy_item_collection_model.yml'
+          examples:
+            'simple response':
+              $ref: '../components/examples/hierarchy_item_collection_response.yml'
+            'filtered response':
+              $ref: '../components/examples/hierarchy_item_collection_filtered_response.yml'
+    '403':
+      description: Returned if the user is not logged in.
+      content:
+        application/hal+json:
+          schema:
+            $ref: '../components/schemas/error_response.yml'
+          examples:
+            response:
+              value:
+                _type: Error
+                errorIdentifier: urn:openproject-org:api:v3:errors:MissingPermission
+                message: You are not authorized to access this resource.
+    '404':
+      description: Returned if the custom field does not exist.
+      content:
+        application/hal+json:
+          schema:
+            $ref: '../components/schemas/error_response.yml'
+          examples:
+            response:
+              value:
+                _type: Error
+                errorIdentifier: urn:openproject-org:api:v3:errors:NotFound
+                message: The requested resource could not be found.
+    '422':
+      description: Returned if the custom field is not of type hierarchy.
+      content:
+        application/hal+json:
+          schema:
+            $ref: '../components/schemas/error_response.yml'
+          examples:
+            response:
+              value:
+                _type: Error
+                errorIdentifier: urn:openproject-org:api:v3:errors:UnprocessableContent
+                message: The requested custom field resource is of wrong type.
diff --git a/lib/api/errors/unprocessable_content.rb b/lib/api/errors/unprocessable_content.rb
new file mode 100644
index 000000000000..2b2eaf5c4e9c
--- /dev/null
+++ b/lib/api/errors/unprocessable_content.rb
@@ -0,0 +1,36 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module API
+  module Errors
+    class UnprocessableContent < ErrorBase
+      identifier "UnprocessableContent"
+      code 422
+    end
+  end
+end
diff --git a/lib/api/v3/custom_fields/custom_fields_api.rb b/lib/api/v3/custom_fields/custom_fields_api.rb
new file mode 100644
index 000000000000..a8332356928d
--- /dev/null
+++ b/lib/api/v3/custom_fields/custom_fields_api.rb
@@ -0,0 +1,47 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module API
+  module V3
+    module CustomFields
+      class CustomFieldsAPI < ::API::OpenProjectAPI
+        resource :custom_fields do
+          route_param :id, type: Integer, desc: "Custom Field ID" do
+            after_validation do
+              authorize_logged_in
+
+              @custom_field = CustomField.find(params[:id])
+            end
+
+            mount API::V3::CustomFields::Hierarchy::ItemsAPI
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/api/v3/custom_fields/hierarchy/get_items_parameter_contract.rb b/lib/api/v3/custom_fields/hierarchy/get_items_parameter_contract.rb
new file mode 100644
index 000000000000..3f7f1bab1fb7
--- /dev/null
+++ b/lib/api/v3/custom_fields/hierarchy/get_items_parameter_contract.rb
@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module API
+  module V3
+    module CustomFields
+      module Hierarchy
+        class GetItemsParameterContract < Dry::Validation::Contract
+          config.messages.backend = :i18n
+
+          option :hierarchy_root
+
+          params do
+            optional(:parent).filled(:integer)
+            optional(:depth).filled(:integer)
+          end
+
+          rule(:parent) do
+            next unless key?
+
+            parent_item = ::CustomField::Hierarchy::Item.find_by(id: value)
+
+            if parent_item.nil?
+              key.failure(:not_found)
+            elsif persistence_service.descendant_of?(item: parent_item, parent: hierarchy_root).failure?
+              key.failure(:not_descendant)
+            end
+          end
+
+          rule(:depth) do
+            next unless key?
+
+            key.failure(:greater_or_equal_zero) if value < 0
+          end
+
+          private
+
+          def persistence_service
+            @persistence_service ||= ::CustomFields::Hierarchy::HierarchicalItemService.new
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/api/v3/custom_fields/hierarchy/hierarchy_item_collection_representer.rb b/lib/api/v3/custom_fields/hierarchy/hierarchy_item_collection_representer.rb
new file mode 100644
index 000000000000..badb8cad2e77
--- /dev/null
+++ b/lib/api/v3/custom_fields/hierarchy/hierarchy_item_collection_representer.rb
@@ -0,0 +1,38 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module API
+  module V3
+    module CustomFields
+      module Hierarchy
+        class HierarchyItemCollectionRepresenter < ::API::Decorators::UnpaginatedCollection
+        end
+      end
+    end
+  end
+end
diff --git a/lib/api/v3/custom_fields/hierarchy/hierarchy_item_representer.rb b/lib/api/v3/custom_fields/hierarchy/hierarchy_item_representer.rb
index bf1c63b26e25..fddbbd2a3f44 100644
--- a/lib/api/v3/custom_fields/hierarchy/hierarchy_item_representer.rb
+++ b/lib/api/v3/custom_fields/hierarchy/hierarchy_item_representer.rb
@@ -35,7 +35,36 @@ def _type
             "HierarchyItem"
           end
 
+          self_link path: :custom_field_item,
+                    title_getter: ->(*) { represented.label }
+
           property :id
+
+          property :label, render_nil: true
+
+          property :short, render_nil: true
+
+          property :depth
+
+          link :parent do
+            next if represented.root?
+
+            parent = represented.parent
+
+            {
+              href: api_v3_paths.custom_field_item(parent.id),
+              title: parent.label
+            }
+          end
+
+          links :children do
+            represented.children.map do |child|
+              {
+                href: api_v3_paths.custom_field_item(child.id),
+                title: child.label
+              }
+            end
+          end
         end
       end
     end
diff --git a/lib/api/v3/custom_fields/hierarchy/item_api.rb b/lib/api/v3/custom_fields/hierarchy/item_api.rb
new file mode 100644
index 000000000000..d0ce2d51b6ae
--- /dev/null
+++ b/lib/api/v3/custom_fields/hierarchy/item_api.rb
@@ -0,0 +1,53 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module API
+  module V3
+    module CustomFields
+      module Hierarchy
+        class ItemAPI < ::API::OpenProjectAPI
+          resource :custom_field_items do
+            route_param :id, type: Integer, desc: "Custom Field Item ID" do
+              after_validation do
+                authorize_logged_in
+
+                @custom_field_item = CustomField::Hierarchy::Item.find(params[:id])
+              end
+
+              get &::API::V3::Utilities::Endpoints::Show
+                     .new(model: CustomField::Hierarchy::Item,
+                          render_representer: HierarchyItemRepresenter,
+                          instance_generator: ->(*) { @custom_field_item })
+                     .mount
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/api/v3/custom_fields/hierarchy/items_api.rb b/lib/api/v3/custom_fields/hierarchy/items_api.rb
new file mode 100644
index 000000000000..101e6fca6008
--- /dev/null
+++ b/lib/api/v3/custom_fields/hierarchy/items_api.rb
@@ -0,0 +1,131 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module API
+  module V3
+    module CustomFields
+      module Hierarchy
+        class ItemsAPI < ::API::OpenProjectAPI
+          include Dry::Monads[:result]
+
+          helpers do
+            def flatten_tree_hash(hash)
+              flat_list = []
+              queue = [hash]
+
+              # From the service we get a hashed tree like this:
+              # {:a => {:b => {:c1 => {:d1 => {}}, :c2 => {:d2 => {}}}, :b2 => {}}}
+              # We flatten it depth first to this result list:
+              # [:a, :b, :c1, :d1, :c2, :d2, :b2]
+
+              while queue.any?
+                current = queue.shift
+                item, children = current.shift
+                flat_list << item
+                queue.unshift(current) unless current.empty?
+                queue.unshift(children) unless children.empty?
+              end
+
+              flat_list
+            end
+
+            def item_list(query)
+              hierarchy_root = get_hierarchy_root(query)
+
+              validation = GetItemsParameterContract.new(hierarchy_root:).call(params)
+              handle_validation_errors(validation) if validation.failure?
+
+              start_item = ::CustomField::Hierarchy::Item.find_by(id: validation[:parent]) || hierarchy_root
+              depth = validation[:depth] || -1
+
+              flat_tree(start_item, depth)
+            end
+
+            def flat_tree(item, depth)
+              sub_tree = ::CustomFields::Hierarchy::HierarchicalItemService
+                           .new
+                           .hashed_subtree(item:, depth:)
+                           .either(
+                             ->(value) { value },
+                             ->(error) do
+                               msg = "#{I18n.t('api_v3.errors.code_500')} #{error}"
+                               raise ::API::Errors::InternalError.new(msg)
+                             end
+                           )
+
+              flatten_tree_hash(sub_tree)
+            end
+
+            def get_hierarchy_root(query)
+              items = query.results.where(custom_field: @custom_field)
+              if items.count != 1
+                msg = "corrupt data found, invalid number of hierarchy roots for custom field"
+                raise ::API::Errors::InternalError.new(msg)
+              end
+
+              items.first
+            end
+
+            def handle_validation_errors(validation)
+              message = ""
+              validation.errors(full: true).to_h.each_value do |value|
+                message += "#{value.join(', ')}\n"
+              end
+
+              raise ::API::Errors::InvalidQuery.new(message.chomp)
+            end
+          end
+
+          resource :items do
+            after_validation do
+              unless @custom_field.field_format_hierarchy?
+                message = "Hierarchy items do only exists for custom fields of type hierarchy."
+                raise ::API::Errors::UnprocessableContent.new(message)
+              end
+            end
+
+            get do
+              query = ParamsToQueryService.new(::CustomField::Hierarchy::Item,
+                                               current_user,
+                                               query_class: ::Queries::CustomFields::Hierarchy::ItemQuery)
+                                          .call(params)
+
+              unless query.valid?
+                message = I18n.t("api_v3.errors.missing_or_malformed_parameter", parameter: "filters")
+                raise ::API::Errors::InvalidQuery.new(message)
+              end
+
+              self_link = api_v3_paths.custom_field_items(@custom_field.id, params[:parent], params[:depth])
+              HierarchyItemCollectionRepresenter.new(item_list(query), self_link:, current_user:)
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/api/v3/root.rb b/lib/api/v3/root.rb
index d299b19fd7a2..a30041f486f1 100644
--- a/lib/api/v3/root.rb
+++ b/lib/api/v3/root.rb
@@ -46,11 +46,13 @@ class Root < ::API::OpenProjectAPI
       mount ::API::V3::Actions::ActionsAPI
       mount ::API::V3::Activities::ActivitiesAPI
       mount ::API::V3::Attachments::AttachmentsAPI
-      mount ::API::V3::Capabilities::CapabilitiesAPI
       mount ::API::V3::Backups::BackupsAPI
+      mount ::API::V3::Capabilities::CapabilitiesAPI
       mount ::API::V3::Categories::CategoriesAPI
       mount ::API::V3::Configuration::ConfigurationAPI
       mount ::API::V3::CustomActions::CustomActionsAPI
+      mount ::API::V3::CustomFields::CustomFieldsAPI
+      mount ::API::V3::CustomFields::Hierarchy::ItemAPI
       mount ::API::V3::CustomOptions::CustomOptionsAPI
       mount ::API::V3::Days::DaysAPI
       mount ::API::V3::Grids::GridsAPI
diff --git a/lib/api/v3/utilities/path_helper.rb b/lib/api/v3/utilities/path_helper.rb
index c3a04f70a83d..1bf20c07963e 100644
--- a/lib/api/v3/utilities/path_helper.rb
+++ b/lib/api/v3/utilities/path_helper.rb
@@ -44,11 +44,13 @@ def self.index(name, path = nil)
               RequestStore.store[:"cached_#{plural_name}"] ||= "#{root}/#{path}"
             end
           end
+
           private_class_method :index
 
           def self.show(name, path = name)
             define_singleton_method(name) { |id| build_path(path, id) }
           end
+
           private_class_method :show
 
           def self.create_form(name)
@@ -56,11 +58,13 @@ def self.create_form(name)
               RequestStore.store[:"cached_create_#{name}_form"] ||= build_path(name, "form")
             end
           end
+
           private_class_method :create_form
 
           def self.update_form(name)
             define_singleton_method(:"#{name}_form") { |id| build_path(name, id, "form") }
           end
+
           private_class_method :update_form
 
           def self.schema(name)
@@ -68,11 +72,13 @@ def self.schema(name)
               RequestStore.store[:"cached_#{name}_schema"] ||= build_path(name, "schema")
             end
           end
+
           private_class_method :schema
 
           def self.build_path(name, *kwargs)
             [root, name.to_s.pluralize, *kwargs].compact.join("/")
           end
+
           private_class_method :build_path
 
           def self.resources(name,
@@ -83,6 +89,7 @@ def self.resources(name,
               send(method, name)
             end
           end
+
           private_class_method :resources
 
           # Determining the root_path on every url we want to render is
@@ -200,6 +207,24 @@ def self.custom_action_execute(id)
             "#{custom_action(id)}/execute"
           end
 
+          def self.custom_field(id)
+            "#{root}/custom_fields/#{id}"
+          end
+
+          def self.custom_field_item(id)
+            "#{root}/custom_field_items/#{id}"
+          end
+
+          def self.custom_field_items(id, parent = nil, depth = nil)
+            query = { parent:, depth: }.compact_blank.to_query
+
+            if query.present?
+              "#{custom_field(id)}/items?#{query}"
+            else
+              "#{custom_field(id)}/items"
+            end
+          end
+
           def self.custom_option(id)
             "#{root}/custom_options/#{id}"
           end
@@ -511,9 +536,9 @@ def self.wiki_page(id)
 
           def self.work_package(id, timestamps: nil)
             "#{root}/work_packages/#{id}" + \
-            if (param_value = timestamps_to_param_value(timestamps)).present? && Array(timestamps).any?(&:historic?)
-              "?#{{ timestamps: param_value }.to_query}"
-            end.to_s
+              if (param_value = timestamps_to_param_value(timestamps)).present? && Array(timestamps).any?(&:historic?)
+                "?#{{ timestamps: param_value }.to_query}"
+              end.to_s
           end
 
           def self.work_package_schema(project_id, type_id)
diff --git a/spec/lib/api/v3/utilities/resource_link_generator_spec.rb b/spec/lib/api/v3/utilities/resource_link_generator_spec.rb
index b9c0f035527c..867e0f3ae498 100644
--- a/spec/lib/api/v3/utilities/resource_link_generator_spec.rb
+++ b/spec/lib/api/v3/utilities/resource_link_generator_spec.rb
@@ -55,7 +55,7 @@
     end
 
     it "returns nil for unsupported records" do
-      record = create(:custom_field)
+      record = create(:oauth_client_token)
       expect(subject.make_link(record)).to be_nil
     end
 
diff --git a/spec/services/custom_fields/hierarchy/hierarchical_item_service_spec.rb b/spec/services/custom_fields/hierarchy/hierarchical_item_service_spec.rb
index e1266f3e6497..b75bad74bccb 100644
--- a/spec/services/custom_fields/hierarchy/hierarchical_item_service_spec.rb
+++ b/spec/services/custom_fields/hierarchy/hierarchical_item_service_spec.rb
@@ -180,7 +180,7 @@
     end
   end
 
-  describe "reorder_item" do
+  describe "#reorder_item" do
     let!(:lando) { service.insert_item(parent: root, label: "lando").value! }
     let!(:chewbacca) { service.insert_item(parent: root, label: "AWOOO").value! }
 
@@ -240,4 +240,26 @@
       expect(chewbacca.reload.sort_order).to eq(2)
     end
   end
+
+  describe "#hashed_subtree" do
+    let!(:lando) { service.insert_item(parent: root, label: "lando").value! }
+    let!(:chewbacca) { service.insert_item(parent: root, label: "AWOOO").value! }
+    let!(:lowbacca) { service.insert_item(parent: chewbacca, label: "ARWWWW").value! }
+
+    it "produces a hash version of the tree" do
+      subtree = service.hashed_subtree(item: root, depth: -1)
+
+      expect(subtree.value!).to be_a(Hash)
+      expect(subtree.value![root].size).to eq(3)
+      expect(subtree.value![root][lando]).to be_empty
+      expect(subtree.value![root][chewbacca][lowbacca]).to be_empty
+    end
+
+    it "produces a hash version of a sub tree with limited depth" do
+      subtree = service.hashed_subtree(item: chewbacca, depth: 0)
+
+      expect(subtree.value!).to be_a(Hash)
+      expect(subtree.value![chewbacca]).to be_empty
+    end
+  end
 end