diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index bd14824..7ddea1f 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -156,6 +156,131 @@ jobs:
           test -f debian/artifacts/test_1_amd64.buildinfo
           test -f debian/artifacts/test_1_amd64.changes
 
+  extra-repo-keys-ascii:
+    needs: [single-package]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - id: download-keys
+        run: |
+          docker run --detach --mount="type=bind,src=${{ github.workspace }},dst=/workspace" --name=download-cont --rm --workdir=/workspace debian:stable-slim tail -f /dev/null
+          docker exec download-cont apt-get update
+          docker exec --user="$(id -u):$(id -g)" download-cont apt-get download debian-archive-keyring
+          docker stop --time=1 download-cont
+          dpkg-deb --extract debian-archive-keyring_*.deb debian-archive-keyring
+          {
+            printf 'debian-archive-keys<<EOF\n'
+            cat debian-archive-keyring/etc/apt/trusted.gpg.d/*.asc
+            printf 'EOF\n'
+          } >>"$GITHUB_OUTPUT"
+      - uses: ./
+        env:
+          DEB_BUILD_OPTIONS: noautodbgsym
+        with:
+          buildpackage-opts: --build=binary --no-sign
+          docker-image: test/Dockerfile_extra-repo-keys
+          # An output value instead of an environment variable is used here to
+          # not interfere with Docker’s environment down the line.
+          extra-repo-keys: ${{ steps.download-keys.outputs.debian-archive-keys }}
+          source-dir: test
+      - run: |
+          dpkg --info debian/artifacts/test_1_amd64.deb
+          dpkg --contents debian/artifacts/test_1_amd64.deb | grep ./usr/bin/mybin
+          test -f debian/artifacts/test_1_amd64.buildinfo
+          test -f debian/artifacts/test_1_amd64.changes
+
+  extra-repo-keys-files:
+    needs: [single-package]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - id: download-keys
+        run: |
+          docker run --detach --mount="type=bind,src=${{ github.workspace }},dst=/workspace" --name=download-cont --rm --workdir=/workspace debian:stable-slim tail -f /dev/null
+          docker exec download-cont apt-get update
+          docker exec --user="$(id -u):$(id -g)" download-cont apt-get download debian-archive-keyring
+          docker stop --time=1 download-cont
+          dpkg-deb --extract debian-archive-keyring_*.deb debian-archive-keyring
+          {
+            printf 'debian-archive-keys<<EOF\n'
+            ls -1 --directory debian-archive-keyring/etc/apt/trusted.gpg.d/*.asc
+            printf 'EOF\n'
+          } >>"$GITHUB_OUTPUT"
+      - uses: ./
+        env:
+          DEB_BUILD_OPTIONS: noautodbgsym
+        with:
+          buildpackage-opts: --build=binary --no-sign
+          docker-image: test/Dockerfile_extra-repo-keys
+          # An output value instead of an environment variable is used here to
+          # not interfere with Docker’s environment down the line.
+          extra-repo-keys: ${{ steps.download-keys.outputs.debian-archive-keys }}
+          source-dir: test
+      - run: |
+          dpkg --info debian/artifacts/test_1_amd64.deb
+          dpkg --contents debian/artifacts/test_1_amd64.deb | grep ./usr/bin/mybin
+          test -f debian/artifacts/test_1_amd64.buildinfo
+          test -f debian/artifacts/test_1_amd64.changes
+
+  extra-repos:
+    needs: [setup-hook]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: cat test/Makefile_extra-repos >>test/Makefile
+      - uses: ./
+        env:
+          DEB_BUILD_OPTIONS: noautodbgsym
+        with:
+          buildpackage-opts: --build=binary --no-sign
+          extra-repos: |
+            # These are intentionally given as two entries:
+            deb http://deb.debian.org/debian stable contrib
+            deb http://deb.debian.org/debian stable non-free
+          setup-hook: |
+            apt-get update # Called here manually only for testing purposes!
+            ! apt-cache policy | grep -E ' *release.*[ ,]o=Debian[,$$]' | grep -E '[ ,]c=contrib[,$$]'
+            ! apt-cache policy | grep -E ' *release.*[ ,]o=Debian[,$$]' | grep -E '[ ,]c=non-free[,$$]'
+          source-dir: test
+      - run: |
+          dpkg --info debian/artifacts/test_1_amd64.deb
+          dpkg --contents debian/artifacts/test_1_amd64.deb | grep ./usr/bin/mybin
+          test -f debian/artifacts/test_1_amd64.buildinfo
+          test -f debian/artifacts/test_1_amd64.changes
+
+  extra-repos-deb822:
+    needs: [setup-hook]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: cat test/Makefile_extra-repos >>test/Makefile
+      - uses: ./
+        env:
+          DEB_BUILD_OPTIONS: noautodbgsym
+        with:
+          buildpackage-opts: --build=binary --no-sign
+          extra-repos: |
+            # These are intentionally given as two entries:
+            Types: deb
+            URIs: http://deb.debian.org/debian
+            Suites: stable
+            Components: contrib
+
+            Types: deb
+            URIs: http://deb.debian.org/debian
+            Suites: stable
+            Components: non-free
+          setup-hook: |
+            apt-get update # Called here manually only for testing purposes!
+            ! apt-cache policy | grep -E ' *release.*[ ,]o=Debian[,$$]' | grep -E '[ ,]c=contrib[,$$]'
+            ! apt-cache policy | grep -E ' *release.*[ ,]o=Debian[,$$]' | grep -E '[ ,]c=non-free[,$$]'
+          source-dir: test
+      - run: |
+          dpkg --info debian/artifacts/test_1_amd64.deb
+          dpkg --contents debian/artifacts/test_1_amd64.deb | grep ./usr/bin/mybin
+          test -f debian/artifacts/test_1_amd64.buildinfo
+          test -f debian/artifacts/test_1_amd64.changes
+
   full-build:
     runs-on: ubuntu-latest
     steps:
diff --git a/README.md b/README.md
index 90c7f27..5bf4ab9 100644
--- a/README.md
+++ b/README.md
@@ -95,6 +95,25 @@ settings are required.
 
 Optional and empty by default.
 
+#### `extra-repos`
+Extra APT repositories to configure as sources in the build environment.
+
+Entries can be given in either format supported by APT: one-line style or
+deb822 style, see
+[`man sources.list`](https://manpages.debian.org/sources.list.5).
+
+Optional and empty by default.
+
+#### `extra-repo-keys`
+Extra keys for APT to trust in the build environment. Useful in combination
+with [`extra-repos`](#extra-repos).
+
+The parameter can be used to pass either one or multiple ASCII-armored keys, or
+a newline-separated list of paths to key files in ASCII-armored or binary
+format. Paths to key files must be relative to the workspace.
+
+Optional and empty by default.
+
 #### `host-arch`
 The architecture packages are built for. If this parameter is set,
 cross-compilation is set up with `apt-get` and `dpkg-buildpackage` as described
diff --git a/action.yml b/action.yml
index af74cf5..ba384ed 100644
--- a/action.yml
+++ b/action.yml
@@ -26,6 +26,12 @@ inputs:
   extra-docker-args:
     description: Additional arguments to 'docker run' when starting the build container
     required: false
+  extra-repo-keys:
+    description: Extra ASCII-armored APT repository keys or paths to keyring files to trust in the build environment
+    required: false
+  extra-repos:
+    description: Extra APT repositories to configure in the build environment (one-line-style or deb822-style format)
+    required: false
   host-arch:
     description: Foreign architecture to setup cross-compilation for
     required: false
@@ -50,6 +56,8 @@ runs:
         INPUT_DOCKER_IMAGE: ${{ inputs.docker-image }}
         INPUT_EXTRA_BUILD_DEPS: ${{ inputs.extra-build-deps }}
         INPUT_EXTRA_DOCKER_ARGS: ${{ inputs.extra-docker-args }}
+        INPUT_EXTRA_REPO_KEYS: ${{  inputs.extra-repo-keys }}
+        INPUT_EXTRA_REPOS: ${{  inputs.extra-repos }}
         INPUT_HOST_ARCH: ${{ inputs.host-arch }}
         INPUT_SETUP_HOOK: ${{ inputs.setup-hook }}
         INPUT_SOURCE_DIR: ${{ inputs.source-dir }}
diff --git a/scripts/install_build_deps b/scripts/install_build_deps
index 3e49aa8..c6f8700 100755
--- a/scripts/install_build_deps
+++ b/scripts/install_build_deps
@@ -24,6 +24,13 @@ fi
 # essential packages. It is reasonably safe to blindly assume it is installed.
 printf "man-db man-db/auto-update boolean false\n" | debconf-set-selections
 
+if printf "%s\n" "$INPUT_EXTRA_REPOS" | grep -q '^deb'; then
+	extra_repos_ext=.list
+else
+	extra_repos_ext=.sources
+fi
+printf "%s\n" "$INPUT_EXTRA_REPOS" >"/etc/apt/sources.list.d/build-deb-action${extra_repos_ext}"
+
 apt-get update
 
 # shellcheck disable=SC2086
diff --git a/scripts/run b/scripts/run
index 5840827..fda7230 100755
--- a/scripts/run
+++ b/scripts/run
@@ -46,6 +46,8 @@ build_dir=$(mktemp --directory --tmpdir="${RUNNER_TEMP-}" build-deb-action-XXXXX
 trap clean_up EXIT INT HUP TERM
 
 env_file=$build_dir/env
+extra_repo_keys_file=$build_dir/build-deb-action.asc
+extra_repo_key_files_file=$build_dir/extra_repo_key_files
 gitconfig_file=$build_dir/gitconfig
 
 INPUT_ARTIFACTS_DIR=${INPUT_ARTIFACTS_DIR:-.}
@@ -60,6 +62,36 @@ if ! check_path_prefix "$INPUT_SOURCE_DIR" "$GITHUB_WORKSPACE"; then
 	exit 2
 fi
 
+touch "$extra_repo_key_files_file"
+if [ "$INPUT_EXTRA_REPO_KEYS" ]; then
+	printf '%s\n' "$INPUT_EXTRA_REPO_KEYS" >"$extra_repo_keys_file"
+
+	while read -r extra_repo_key_file; do
+		[ "$extra_repo_key_file" ] || continue
+
+		if [ ! -f "$extra_repo_key_file" ]; then
+			if [ -s "$extra_repo_key_files_file" ]; then
+				error "extra-repo-keys cannot contain both ASCII-armored keys and key file paths"
+				exit 2
+			fi
+
+			# Looks like (an) ASCII-armored key(s)
+			break
+		fi
+
+		if ! check_path_prefix "$extra_repo_key_file" "$GITHUB_WORKSPACE"; then
+			error "A path to a key file in extra-repo-keys is not in GITHUB_WORKSPACE"
+			exit 2
+		fi
+
+		printf '%s\n' "$extra_repo_key_file" >>"$extra_repo_key_files_file"
+	done <"$extra_repo_keys_file"
+
+	if [ ! -s "$extra_repo_key_files_file" ]; then
+		printf '/github/build/build-deb-action.asc\n' >"$extra_repo_key_files_file"
+	fi
+fi
+
 if [ -f "$INPUT_DOCKER_IMAGE" ]; then
 	if ! check_path_prefix "$INPUT_DOCKER_IMAGE" "$GITHUB_WORKSPACE"; then
 		error "docker-image is the path of a Dockerfile but it is not in GITHUB_WORKSPACE"
@@ -77,9 +109,12 @@ if [ -f "$INPUT_DOCKER_IMAGE" ]; then
 fi
 
 start_group "Starting build container"
-# Docker does not like variable values containing newlines in an --env-file.
+# Docker does not like variable values containing newlines in an --env-file, we
+# will pass them separately:
 env \
 	--unset=INPUT_BEFORE_BUILD_HOOK \
+	--unset=INPUT_EXTRA_REPO_KEYS \
+	--unset=INPUT_EXTRA_REPOS \
 	--unset=INPUT_SETUP_HOOK \
 	>"$env_file"
 
@@ -91,6 +126,7 @@ container_id=$(
 		--env-file="$env_file" \
 		--env=GITHUB_ACTION_PATH=/github/action \
 		--env=GITHUB_WORKSPACE=/github/workspace \
+		--env=INPUT_EXTRA_REPOS \
 		--mount="type=bind,src=${GITHUB_ACTION_PATH},dst=/github/action,ro" \
 		--mount="type=bind,src=${build_dir},dst=/github/build" \
 		--mount="type=bind,src=${GITHUB_WORKSPACE},dst=/github/workspace" \
@@ -103,6 +139,13 @@ container_id=$(
 end_group
 
 start_group "Setting up build container"
+while read -r extra_repo_key_file; do
+	docker exec "$container_id" install \
+		--mode=644 \
+		--target-directory=/etc/apt/trusted.gpg.d \
+		"$extra_repo_key_file"
+done <"$extra_repo_key_files_file"
+
 # git is always available here outside the build container, in the environment
 # of a run step.
 
diff --git a/test/Dockerfile_extra-repo-keys b/test/Dockerfile_extra-repo-keys
new file mode 100644
index 0000000..efac305
--- /dev/null
+++ b/test/Dockerfile_extra-repo-keys
@@ -0,0 +1,3 @@
+FROM debian:stable-slim
+
+RUN rm /etc/apt/trusted.gpg.d/debian-archive-*.asc
diff --git a/test/Makefile_extra-repos b/test/Makefile_extra-repos
new file mode 100644
index 0000000..26e3ce5
--- /dev/null
+++ b/test/Makefile_extra-repos
@@ -0,0 +1,5 @@
+
+.PHONY: test
+test:
+	apt-cache policy | grep -E ' *release.*[ ,]o=Debian[,$$]' | grep -E '[ ,]c=contrib[,$$]'
+	apt-cache policy | grep -E ' *release.*[ ,]o=Debian[,$$]' | grep -E '[ ,]c=non-free[,$$]'