diff --git a/.github/workflows/test_actions.yml b/.github/workflows/test_actions.yml
index 8e1f79a..52e4a18 100644
--- a/.github/workflows/test_actions.yml
+++ b/.github/workflows/test_actions.yml
@@ -254,6 +254,63 @@ jobs:
           project-directory: test-project
           expected-version: 1.0.2.dev1
 
+  test_check_project_links:
+    name: Test check-project-links
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repo
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          persist-credentials: false
+      - name: Create project with a valid link
+        run: |
+          mkdir -p test-project
+          cat > test-project/pyproject.toml <<'EOF'
+          [project]
+          name = "test-project"
+          version = "0.1.0"
+          description = "Test project with a valid URL"
+          authors = [{ name = "NI" }]
+          readme = "README.md"
+          requires-python = ">=3.9"
+
+          [project.urls]
+          Homepage = "https://github.com/ni/python-actions"
+          Documentation = "https://github.com/ni/python-actions/blob/main/README.md"
+          EOF
+        shell: bash
+      - name: Check valid project links
+        uses: ./check-project-links
+        with:
+          project-directory: test-project
+      - name: Create project with a missing link
+        run: |
+          mkdir -p failing-project
+          cat > failing-project/pyproject.toml <<'EOF'
+          [project]
+          name = "failing-project"
+          version = "0.1.0"
+          description = "Test project with a 404 URL"
+          authors = [{ name = "NI" }]
+          readme = "README.md"
+          requires-python = ">=3.9"
+
+          [project.urls]
+          Broken = "https://github.com/ni/project-that-does-not-exist"
+          EOF
+        shell: bash
+      - name: Check missing project link (expected to fail)
+        id: expected-failure
+        continue-on-error: true
+        uses: ./check-project-links
+        with:
+          project-directory: failing-project
+      - name: Error if the previous step didn't fail
+        if: steps.expected-failure.outcome != 'failure'
+        run: |
+          echo "::error title=Test Failure::The previous step did not fail as expected."
+          exit 1
+
   test_analyze_project:
     name: Test analyze-project
     runs-on: ${{ matrix.os }}
@@ -454,6 +511,7 @@ jobs:
       test_setup_poetry_no_cache,
       test_check_project_version,
       test_update_project_version,
+      test_check_project_links,
       test_analyze_project,
       test_analyze_project_repo_root,
     ]
diff --git a/README.md b/README.md
index b134f3b..f760978 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,9 @@
 - [`ni/python-actions/check-project-version`](check-project-version): uses Poetry to get the version
   of a Python project and checks that it matches an expected version. Publish workflows can use this
   to verify that the release tag matches the version number in `pyproject.toml`.
+- [`ni/python-actions/check-project-links`](check-project-links): scans project `pyproject.toml` files
+  for URLs, writes the discovered links to a temporary file, and validates each URL in Docker while
+  failing only on `4xx` responses.
 - [`ni/python-actions/update-project-version`](update-project-version): uses Poetry to update the
   version of a Python project and creates a pull request to modify its `pyproject.toml` file.
   Publish workflows can use this to update the version in `pyproject.toml` for the next build.
diff --git a/check-project-links/README.md b/check-project-links/README.md
new file mode 100644
index 0000000..6a03ff7
--- /dev/null
+++ b/check-project-links/README.md
@@ -0,0 +1,55 @@
+# `ni/python-actions/check-project-links`
+
+This action searches a project tree for `pyproject.toml` files, extracts `https://` links from those files, and validates each discovered URL by making an HTTP request in a Docker container. It fails only when a checked URL returns a `4xx` response. `2xx` and `3xx` responses are treated as successful and logged without failing the action.
+
+## Inputs
+
+### `project-directory`
+
+Path to the directory containing one or more `pyproject.toml` files.
+
+Default: `${{ github.workspace }}`
+
+### `allowed-domains`
+
+Comma-separated list of trusted hostnames or domains to validate. Supports wildcards like `*.readthedocs.io`.
+
+Default: `github.com,ni.github.io,*.readthedocs.io`
+
+### `docker-image`
+
+Docker image used to perform the HTTP requests.
+
+Default: `curlimages/curl:8.22.0@sha256:58adaa4e8dca9c988bae2aba4ab3434a0bb2da16bbe3f92dec39ec7785166777`
+
+> [!NOTE]
+> The action default uses a full digest SHA, though this is not required.
+## Examples
+
+> [!NOTE]
+> These examples use `@v0`, but pinning to a commit hash or full release tag is recommended for
+> build reproducibility and security.
+
+
+```yaml
+steps:
+  - uses: actions/checkout@v0
+
+  - name: Check project links
+    uses: ni/python-actions/check-project-links@v0
+    with:
+      project-directory: .
+      docker-image: curlimages/curl:8.22.0
+```
+
+## Behavior
+
+- Uses Python directory walk + regex search to identify `https://` links in `pyproject.toml` files under the provided path.
+- Extracts `https://` substrings (including from comments) and filters them by the configured allowed domains before validation.
+- Drops any URL whose hostname is `localhost`, a local loopback address, or any literal IP address before validation.
+- Deduplicates the list of URLs and writes them to a temporary file.
+- Validates each URL with the configured Docker image.
+- Fails immediately if `docker` is not installed or available on `PATH`.
+- Logs `2xx` and `3xx` responses as passing.
+- Fails the action only when a URL returns a `4xx` status code.
+- Other status codes are considered a warning.
diff --git a/check-project-links/_find_project_links.py b/check-project-links/_find_project_links.py
new file mode 100644
index 0000000..5b99398
--- /dev/null
+++ b/check-project-links/_find_project_links.py
@@ -0,0 +1,135 @@
+#!/usr/bin/env python3
+
+import argparse
+import ipaddress
+import os
+import re
+import sys
+from urllib.parse import urlsplit
+
+
+def _parse_args() -> argparse.Namespace:
+    parser = argparse.ArgumentParser(
+        description="Collect trusted project URLs from pyproject.toml files."
+    )
+    parser.add_argument(
+        "project_directory", help="Directory containing project pyproject.toml files."
+    )
+    parser.add_argument("allowed_domains", help="Comma-separated list of allowed domains.")
+    parser.add_argument("output_path", help="Path to write discovered URLs.")
+    return parser.parse_args()
+
+
+def _is_allowed(hostname: str, allowed: set[str]) -> bool:
+    """Check if the given hostname is allowed based on the provided set of allowed domains.
+
+    >>> _is_allowed('example.com', {'example.com'})
+    True
+
+    >>> _is_allowed('sub.example.com', {'example.com'})
+    False
+
+    >>> _is_allowed('sub.example.com', {'*.example.com'})
+    True
+    """
+    if not hostname:
+        return False
+
+    host = hostname.lower().rstrip(".")
+    if host == "localhost" or host.endswith(".localhost"):
+        return False
+
+    try:
+        ip = ipaddress.ip_address(host)
+    except ValueError:
+        ip = None
+
+    if ip is not None:
+        return False
+
+    if host in allowed:
+        return True
+
+    if any(
+        host.endswith(f'.{domain.lstrip(".*")}') for domain in allowed if domain.startswith("*.")
+    ):
+        return True
+
+    return False
+
+
+def _safe_under(base_dir: str, candidate_path: str) -> str:
+    """Return a canonical path that stays under base_dir, or raise ValueError."""
+    safe_base = os.path.realpath(base_dir)
+    safe_candidate = os.path.realpath(candidate_path)
+
+    try:
+        if os.path.commonpath([safe_base, safe_candidate]) != safe_base:
+            raise ValueError(f"Path escapes base directory: {candidate_path!r}")
+    except ValueError as exc:
+        raise ValueError(f"Path escapes base directory: {candidate_path!r}") from exc
+
+    return safe_candidate
+
+
+_URL_RE = re.compile(r'(https://.+?)(?:"|\s)')
+
+
+def _extract_links_from_line(line: str) -> list[str]:
+    """Extract URLs from a single line, stopping before a quote or whitespace."""
+    matches: list[str] = []
+    for match in re.finditer(_URL_RE, line):
+        url = match.group(1)
+        if url:
+            matches.append(url)
+    return matches
+
+
+def _find_project_links(manifest_path: str, results: set[str], allowed: set[str]) -> None:
+    """Find URLs within a pyproject.toml file without parsing TOML."""
+    try:
+        with open(manifest_path, "r", encoding="utf-8") as manifest_file:
+            for line in manifest_file:
+                for url in _extract_links_from_line(line):
+                    host = urlsplit(url).hostname
+                    if host and _is_allowed(host, allowed):
+                        results.add(url)
+    except OSError:
+        print(f"Warning: Failed to read pyproject.toml at {manifest_path}", file=sys.stderr)
+
+
+def _main() -> int:
+    args = _parse_args()
+    safe_project_directory = _safe_under(os.getcwd(), args.project_directory)
+    allowed_domains = args.allowed_domains
+    output_path = args.output_path
+
+    allowed: set[str] = set()
+    for raw_domain in allowed_domains.split(","):
+        domain = raw_domain.strip().lower().rstrip(".")
+        if domain:
+            allowed.add(domain)
+
+    results: set[str] = set()
+
+    for root, _, files in os.walk(safe_project_directory):
+        for file_name in files:
+            if file_name != "pyproject.toml":
+                continue
+
+            manifest_path = _safe_under(safe_project_directory, os.path.join(root, file_name))
+            _find_project_links(manifest_path, results, allowed)
+
+    output_directory = os.path.dirname(output_path)
+    if output_directory:
+        os.makedirs(output_directory, exist_ok=True)
+
+    with open(output_path, "w", encoding="utf-8") as output_file:
+        for url in sorted(results):
+            output_file.write(f"{url}\n")
+
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(_main())
diff --git a/check-project-links/action.yml b/check-project-links/action.yml
new file mode 100644
index 0000000..2055a9c
--- /dev/null
+++ b/check-project-links/action.yml
@@ -0,0 +1,83 @@
+name: Check project links
+description: Find URLs referenced in pyproject.toml files and fail only when a response is 4xx.
+
+inputs:
+  project-directory:
+    description: Path to the directory containing pyproject.toml files.
+    default: ${{ github.workspace }}
+  allowed-domains:
+    description: Comma-separated list of trusted hostnames or domains to validate. Supports wildcards like *.readthedocs.io.
+    default: github.com,ni.github.io,*.readthedocs.io
+  docker-image:
+    description: Docker image used to validate each discovered URL.
+    default: curlimages/curl:8.22.0@sha256:58adaa4e8dca9c988bae2aba4ab3434a0bb2da16bbe3f92dec39ec7785166777
+
+runs:
+  using: composite
+  steps:
+    - name: Check project links
+      id: check-project-links
+      shell: bash
+      env:
+        PROJECT_DIRECTORY: ${{ inputs.project-directory }}
+        ALLOWED_DOMAINS: ${{ inputs.allowed-domains }}
+        DOCKER_IMAGE: ${{ inputs.docker-image }}
+      run: |
+        set -euo pipefail
+
+        if [ ! -d "$PROJECT_DIRECTORY" ]; then
+          echo "::error title=Check Project Links Error::Project directory '$PROJECT_DIRECTORY' does not exist."
+          exit 1
+        fi
+        
+        if ! command -v docker >/dev/null 2>&1; then
+          echo "::error title=Check Project Links Error::docker is not available. Install Docker or add a pre-step that installs it before using this action."
+          exit 1
+        fi
+
+        link_file="$(mktemp -p "$RUNNER_TEMP")"
+        cleanup() {
+          rm -f "$link_file"
+        }
+        trap cleanup EXIT
+
+        chmod 644 "$link_file" # make the file world-readable so that the unprivileged user in the docker container can read it
+
+        python3 "$GITHUB_ACTION_PATH/_find_project_links.py" "$PROJECT_DIRECTORY" "$ALLOWED_DOMAINS" "$link_file"
+
+        if [ ! -s "$link_file" ]; then
+          echo "No trusted project links found under $PROJECT_DIRECTORY."
+          exit 0
+        fi
+
+        echo "Found $(wc -l < "$link_file") unique trusted links:"
+        cat "$link_file"
+
+        unprivileged_user=100:100 # this matches the default, but we want to be explicit about it
+        docker run -u "${unprivileged_user}" --rm \
+          -v "$link_file:/tmp/project_links.txt:ro" \
+          "$DOCKER_IMAGE" \
+          sh -ec '
+            failed=0
+            while IFS= read -r url; do
+              [ -n "$url" ] || continue
+              status=$(curl -L -sS --connect-timeout 5 --max-time 20 -o /tmp/link_body -w "%{http_code}" "$url" || true)
+              case "$status" in
+                2??|3??)
+                  echo "PASS $url -> $status"
+                  ;;
+                4??)
+                  echo "FAIL $url -> $status"
+                  failed=1
+                  ;;
+                *)
+                  echo "WARN $url -> $status"
+                  ;;
+              esac
+            done < /tmp/project_links.txt
+            exit "$failed"
+          '
+
+        # `docker run` failures already cause this step to fail due to `set -e`.
+
+        echo "Success: No checked project links returned 4xx responses."