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..8c1abe35d368
--- /dev/null
+++ b/docs/api/apiv3/components/examples/hierarchy_item_collection_filtered_response.yml
@@ -0,0 +1,53 @@
+description: |-
+  Filtered response of a hierarchy structure starting at a specific parent and restricting the depth.
+value:
+  _type: Collection
+  total: 2
+  count: 2
+  _embedded:
+    elements:
+      - _type: HierarchyItem
+        id: 1337
+        label: null
+        short: null
+        _links:
+          self:
+            href: /api/v3/custom_field/42/items/1337
+          parent:
+            href: null
+          children:
+            - href: /api/v3/custom_field/42/items/1338
+              name: Stormtroopers
+            - href: /api/v3/custom_field/42/items/1339
+              name: Dark Troopers
+      - _type: HierarchyItem
+        id: 1338
+        label: Stormtroopers
+        short: ST
+        _links:
+          self:
+            href: /api/v3/custom_field/42/items/1338
+          parent:
+            href: /api/v3/custom_field/42/items/1337
+          children:
+            - href: /api/v3/custom_field/42/items/1340
+              name: ST-377
+            - href: /api/v3/custom_field/42/items/1341
+              name: ST-422
+      - _type: HierarchyItem
+        id: 1339
+        label: Dark Troopers
+        short: DT
+        _links:
+          self:
+            href: /api/v3/custom_field/42/items/1339
+          parent:
+            href: /api/v3/custom_field/42/items/1337
+          children:
+            - href: /api/v3/custom_field/42/items/1342
+              name: DT-3817992
+            - href: /api/v3/custom_field/42/items/1343
+              name: DT-0129755
+  _links:
+    self:
+      href: /api/v3/custom_field/42/items?parent=1337&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..ac6a9683d3c4
--- /dev/null
+++ b/docs/api/apiv3/components/examples/hierarchy_item_collection_response.yml
@@ -0,0 +1,45 @@
+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
+        _links:
+          self:
+            href: /api/v3/custom_field/42/items/1337
+          parent:
+            href: null
+          children:
+            - href: /api/v3/custom_field/42/items/1338
+              name: Stormtroopers
+            - href: /api/v3/custom_field/42/items/1339
+              name: Dark Troopers
+      - _type: HierarchyItem
+        id: 1338
+        label: Stormtroopers
+        short: ST
+        _links:
+          self:
+            href: /api/v3/custom_field/42/items/1338
+          parent:
+            href: /api/v3/custom_field/42/items/1337
+          children:
+            - href: /api/v3/custom_field/42/items/1340
+              name: ST-377
+            - href: /api/v3/custom_field/42/items/1341
+              name: ST-422
+      - _type: HierarchyItem
+        _hint: hierarchy item shortened for brevity
+        id: 1340
+        label: ST-377
+        short: null
+      - _hint: hierarchy item shortened for brevity
+  _links:
+    self:
+      href: /api/v3/custom_field/42/items
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..302c21756e8c
--- /dev/null
+++ b/docs/api/apiv3/components/schemas/hierarchy_item_read_model.yml
@@ -0,0 +1,43 @@
+# 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
+  _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:
+            - $ref: './link.yml'
+            - description: |-
+                A hierarchy item that is a child of the current hierarchy item
+                
+                **Resource**: HierarchyItem
diff --git a/docs/api/apiv3/openapi-spec.yml b/docs/api/apiv3/openapi-spec.yml
index 9175c3f08b8b..1865613e7854 100644
--- a/docs/api/apiv3/openapi-spec.yml
+++ b/docs/api/apiv3/openapi-spec.yml
@@ -186,6 +186,8 @@ paths:
     "$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_options/{id}":
     "$ref": "./paths/custom_option.yml"
   "/api/v3/days/non_working":
@@ -489,6 +491,10 @@ 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"
     MembershipCreateRequestCustomMessage:
       $ref: "./components/examples/membership-create-request-custom-message.yml"
     MembershipCreateRequestGlobalRole:
@@ -673,6 +679,10 @@ components:
       "$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:
@@ -946,4 +956,4 @@ tags:
   - "$ref": "./tags/work_packages.yml"
   - "$ref": "./tags/work_schedule.yml"
 security:
-  - BasicAuth: []
+  - BasicAuth: [ ]
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..4e47755bbc3e
--- /dev/null
+++ b/docs/api/apiv3/paths/custom_field_items.yml
@@ -0,0 +1,71 @@
+# /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.
+    
+    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'
+    '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..95537b8bd732
--- /dev/null
+++ b/lib/api/errors/unprocessable_content.rb
@@ -0,0 +1,43 @@
+#-- 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
+
+      def initialize(property, message)
+        super(message)
+
+        @property = property
+        @details = { attribute: property }
+      end
+    end
+  end
+end