diff --git a/.github/workflows/test-validate.yaml b/.github/workflows/test-validate.yaml
new file mode 100644
index 0000000..4d8ce4c
--- /dev/null
+++ b/.github/workflows/test-validate.yaml
@@ -0,0 +1,81 @@
+name: validate
+on:
+  pull_request:
+  push:
+jobs:
+  oasdiff_validate_valid:
+    runs-on: ubuntu-latest
+    name: Test validate on a valid spec
+    steps:
+      - name: checkout
+        uses: actions/checkout@v6
+      - name: Running validate action on a valid spec
+        id: test_validate_valid
+        uses: ./validate
+        with:
+          spec: 'specs/valid.yaml'
+      - name: Test validate reports zero findings
+        run: |
+          findings="${{ steps.test_validate_valid.outputs.findings }}"
+          if [ "$findings" != "0" ]; then
+            echo "Expected 0 findings, got '$findings'" >&2
+            exit 1
+          fi
+  oasdiff_validate_findings:
+    runs-on: ubuntu-latest
+    name: Test validate fails on an invalid spec
+    steps:
+      - name: checkout
+        uses: actions/checkout@v6
+      - name: Running validate action on an invalid spec
+        id: test_validate_findings
+        continue-on-error: true
+        uses: ./validate
+        with:
+          spec: 'specs/invalid.yaml'
+      - name: Test validate failed and reported findings
+        run: |
+          if [ "${{ steps.test_validate_findings.outcome }}" != "failure" ]; then
+            echo "Expected the validate step to fail on error-level findings" >&2
+            exit 1
+          fi
+          findings="${{ steps.test_validate_findings.outputs.findings }}"
+          if [ "$findings" = "0" ] || [ -z "$findings" ]; then
+            echo "Expected findings > 0, got '$findings'" >&2
+            exit 1
+          fi
+  oasdiff_validate_fail_on:
+    runs-on: ubuntu-latest
+    name: Test validate severity threshold (--fail-on)
+    steps:
+      - name: checkout
+        uses: actions/checkout@v6
+      - name: Validate a warning-only spec with the default threshold
+        id: test_validate_warn_default
+        uses: ./validate
+        with:
+          spec: 'specs/validate-warning.yaml'
+      - name: Default threshold reports the warning but passes
+        run: |
+          if [ "${{ steps.test_validate_warn_default.outcome }}" != "success" ]; then
+            echo "Expected the step to pass (warnings don't fail by default)" >&2
+            exit 1
+          fi
+          warnings="${{ steps.test_validate_warn_default.outputs.warning_count }}"
+          if [ "$warnings" != "1" ]; then
+            echo "Expected warning_count 1, got '$warnings'" >&2
+            exit 1
+          fi
+      - name: Validate the same spec with fail-on WARN
+        id: test_validate_warn_failon
+        continue-on-error: true
+        uses: ./validate
+        with:
+          spec: 'specs/validate-warning.yaml'
+          fail-on: 'WARN'
+      - name: fail-on WARN escalates the warning to a failure
+        run: |
+          if [ "${{ steps.test_validate_warn_failon.outcome }}" != "failure" ]; then
+            echo "Expected the step to fail with fail-on WARN" >&2
+            exit 1
+          fi
diff --git a/README.md b/README.md
index 022e497..4fe905f 100644
--- a/README.md
+++ b/README.md
@@ -3,6 +3,7 @@
 [![changelog](https://github.com/oasdiff/oasdiff-action/actions/workflows/test-changelog.yaml/badge.svg)](https://github.com/oasdiff/oasdiff-action/actions/workflows/test-changelog.yaml)
 [![diff](https://github.com/oasdiff/oasdiff-action/actions/workflows/test-diff.yaml/badge.svg)](https://github.com/oasdiff/oasdiff-action/actions/workflows/test-diff.yaml)
 [![pr-comment](https://github.com/oasdiff/oasdiff-action/actions/workflows/test-pr-comment.yaml/badge.svg)](https://github.com/oasdiff/oasdiff-action/actions/workflows/test-pr-comment.yaml)
+[![validate](https://github.com/oasdiff/oasdiff-action/actions/workflows/test-validate.yaml/badge.svg)](https://github.com/oasdiff/oasdiff-action/actions/workflows/test-validate.yaml)
 
 GitHub Actions for comparing OpenAPI specs and detecting breaking changes, based on [oasdiff](https://github.com/oasdiff/oasdiff).
 
@@ -13,6 +14,7 @@ GitHub Actions for comparing OpenAPI specs and detecting breaking changes, based
   - [Check for breaking changes](#check-for-breaking-changes)
   - [Generate a changelog](#generate-a-changelog)
   - [Generate a diff report](#generate-a-diff-report)
+  - [Validate a single spec](#validate-a-single-spec)
 - [Configuring with `.oasdiff.yaml`](#configuring-with-oasdiffyaml)
 - [Spec paths](#spec-paths)
 - [Pro: Rich PR comment](#pro-rich-pr-comment)
@@ -159,6 +161,33 @@ jobs:
 | `flatten-allof` | `false` | Merge allOf subschemas into a single schema before diff | `true`, `false` |
 | `output-to-file` | `''` | Write output to this file path instead of stdout | file path |
 
+### Validate a single spec
+
+Validates one OpenAPI spec against the OpenAPI and JSON Schema rules and writes an inline GitHub annotation for each finding. Unlike the other actions it takes a single spec, not a base/revision pair. Findings are classified by severity (error, warning, info); by default the workflow fails only on errors.
+
+```yaml
+name: oasdiff
+on:
+  pull_request:
+    branches: [ "main" ]
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: oasdiff/oasdiff-action/validate@v0.0.48
+        with:
+          spec: 'openapi.yaml'
+```
+
+| Input | Default | Description | Accepted values |
+|---|---|---|---|
+| `spec` | — (required) | Path to the OpenAPI spec to validate | file path, URL, git ref |
+| `fail-on` | `''` | Fail with exit code 1 when a finding is at or above this severity (empty uses the oasdiff default, `ERR`) | `ERR`, `WARN`, `INFO` |
+| `allow-external-refs` | `true` | Resolve external `$ref`s; set `false` to prevent SSRF when validating untrusted specs | `true`, `false` |
+
+For a non-blocking, report-only run, leave `fail-on` and set `continue-on-error: true` on the step. Outputs: `findings` (total), `error_count`, `warning_count`, `info_count`.
+
 ---
 
 ## Configuring with `.oasdiff.yaml`
diff --git a/release.sh b/release.sh
index ccfc9b2..a98b7de 100755
--- a/release.sh
+++ b/release.sh
@@ -11,7 +11,7 @@ set -e
 
 REPO_DIR="$(cd "$(dirname "$0")" && pwd)"
 
-DOCKERFILES="breaking/Dockerfile changelog/Dockerfile diff/Dockerfile pr-comment/Dockerfile"
+DOCKERFILES="breaking/Dockerfile changelog/Dockerfile diff/Dockerfile validate/Dockerfile pr-comment/Dockerfile"
 
 # ── Resolve action version ───────────────────────────────────────────────────
 
diff --git a/specs/invalid.yaml b/specs/invalid.yaml
new file mode 100644
index 0000000..f0e851e
--- /dev/null
+++ b/specs/invalid.yaml
@@ -0,0 +1,4 @@
+openapi: 3.0.0
+info:
+  title: invalid
+paths: {}
diff --git a/specs/valid.yaml b/specs/valid.yaml
new file mode 100644
index 0000000..b034c88
--- /dev/null
+++ b/specs/valid.yaml
@@ -0,0 +1,10 @@
+openapi: 3.0.0
+info:
+  title: valid
+  version: "1.0.0"
+paths:
+  /things:
+    get:
+      responses:
+        '200':
+          description: ok
diff --git a/specs/validate-warning.yaml b/specs/validate-warning.yaml
new file mode 100644
index 0000000..a816f09
--- /dev/null
+++ b/specs/validate-warning.yaml
@@ -0,0 +1,8 @@
+openapi: 3.0.0
+info:
+  title: warning
+  version: "1.0.0"
+  license:
+    name: MIT
+    identifier: MIT
+paths: {}
diff --git a/validate/Dockerfile b/validate/Dockerfile
new file mode 100644
index 0000000..d77b893
--- /dev/null
+++ b/validate/Dockerfile
@@ -0,0 +1,4 @@
+FROM tufin/oasdiff:v1.16.0
+ENV PLATFORM github-action
+COPY entrypoint.sh /entrypoint.sh
+ENTRYPOINT ["/entrypoint.sh"]
diff --git a/validate/action.yml b/validate/action.yml
new file mode 100644
index 0000000..b358264
--- /dev/null
+++ b/validate/action.yml
@@ -0,0 +1,30 @@
+name: 'Validate an OpenAPI spec'
+description: 'Validate an OpenAPI spec against the spec, with per-finding PR annotations'
+inputs:
+  spec:
+    description: 'Path of the OpenAPI spec in YAML or JSON format'
+    required: true
+  fail-on:
+    description: 'Fail with exit code 1 when a finding has this severity or higher: ERR, WARN, or INFO. Defaults to ERR (errors fail the build; warnings and info are reported but do not). For a non-blocking, report-only run, set continue-on-error on the step.'
+    required: false
+    default: ''
+  allow-external-refs:
+    description: 'Allow external $refs in the spec; disable to prevent SSRF when validating untrusted specs'
+    required: false
+    default: 'true'
+outputs:
+  findings:
+    description: 'Total number of findings reported by validate (0 if the spec is valid)'
+  error_count:
+    description: 'Number of error-level findings'
+  warning_count:
+    description: 'Number of warning-level findings'
+  info_count:
+    description: 'Number of info-level findings'
+runs:
+  using: 'docker'
+  image: 'Dockerfile'
+  args:
+    - ${{ inputs.spec }}
+    - ${{ inputs.fail-on }}
+    - ${{ inputs.allow-external-refs }}
diff --git a/validate/entrypoint.sh b/validate/entrypoint.sh
new file mode 100755
index 0000000..4899134
--- /dev/null
+++ b/validate/entrypoint.sh
@@ -0,0 +1,65 @@
+#!/bin/sh
+set -e
+
+if [ -n "$GITHUB_WORKSPACE" ]; then
+  git config --global --get-all safe.directory | grep -q "$GITHUB_WORKSPACE" || \
+  git config --global --add safe.directory "$GITHUB_WORKSPACE"
+fi
+
+readonly spec="$1"
+readonly fail_on="$2"
+readonly allow_external_refs="$3"
+
+echo "running oasdiff validate... spec: $spec, fail_on: $fail_on, allow_external_refs: $allow_external_refs"
+
+# Build flags. --allow-external-refs defaults to true in oasdiff, so only
+# pass it when the input opts out. --fail-on defaults to ERR in oasdiff
+# (errors fail the build; warnings and info are reported but don't), so only
+# pass it when the input overrides the threshold.
+flags=""
+if [ "$allow_external_refs" = "false" ]; then
+    flags="$flags --allow-external-refs=false"
+fi
+if [ -n "$fail_on" ]; then
+    flags="$flags --fail-on $fail_on"
+fi
+echo "flags: $flags"
+
+# Run 1: render annotations to stdout via --format githubactions so GitHub
+# parses them onto the PR's "Files changed" tab. This is the authoritative
+# run: its exit code honours --fail-on (1 when a finding is at or above the
+# threshold, 0 otherwise). Tolerate non-zero so we can still set the outputs
+# below; the exit code is reapplied at the end.
+exit_code=0
+oasdiff validate $flags --format githubactions "$spec" || exit_code=$?
+
+# Run 2: text format, captured for the finding count. Tolerate non-zero
+# exit (the authoritative decision is already captured above).
+findings_text=$(oasdiff validate $flags "$spec") || true
+
+# *** GitHub Action step output ***
+
+# Total finding count from the header "N findings: N error, N warning, N info".
+# A valid spec prints nothing, so the count stays 0.
+findings_count=0
+if [ -n "$findings_text" ]; then
+    header=$(printf '%s' "$findings_text" | head -n 1)
+    n=$(printf '%s' "$header" | awk '{print $1}')
+    if printf '%s' "$n" | grep -qE '^[0-9]+$'; then
+        findings_count="$n"
+    fi
+fi
+echo "findings=$findings_count" >>"$GITHUB_OUTPUT"
+
+# The --format githubactions run above writes error_count/warning_count/
+# info_count to GITHUB_OUTPUT, but only when there are findings. Emit zeros
+# for a valid spec so those outputs are always present for callers.
+if [ "$findings_count" -eq 0 ]; then
+    {
+        echo "error_count=0"
+        echo "warning_count=0"
+        echo "info_count=0"
+    } >>"$GITHUB_OUTPUT"
+fi
+
+exit "$exit_code"