diff --git a/.github/pinact.yml b/.github/pinact.yml
new file mode 100644
index 0000000..d128fe0
--- /dev/null
+++ b/.github/pinact.yml
@@ -0,0 +1,11 @@
+# pinact (https://github.com/suzuki-shunsuke/pinact) pins third-party actions to
+# a commit SHA and updates them: `pinact run --update`. See CONTRIBUTING.md.
+version: 3
+# Skip versions published in the last week, like `pnpm dep:check`.
+min_age:
+  value: 7
+rules:
+  # GitHub's own actions stay on version tags (see .github/zizmor.yml).
+  - ignore: true
+    conditions:
+      - expr: ActionRepoOwner in ["actions", "github"]
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 09c12c0..dd501ab 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,6 +12,27 @@ on:
 permissions: {}
 
 jobs:
+  # Security audit of the workflows (.github/zizmor.yml), including that
+  # third-party actions are pinned to a commit SHA. Same config as
+  # `pnpm lint:gha`.
+  lint-gha:
+    name: Lint GitHub Actions
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read # to check out repo (actions/checkout)
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          persist-credentials: false
+      - uses: zizmorcore/zizmor-action@53290badfa369cb0bf7ad16342cb62c2fa47ac7b # v0.8.0
+        with:
+          version: 1.31.1
+          # The default, set explicitly: online audits include impostor-commit,
+          # which checks that each SHA pin is a commit in the action's own repo.
+          online-audits: true
+          advanced-security: false
+          annotations: true
+
   test:
     name: Test on ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
@@ -96,9 +117,11 @@ jobs:
           cache: 'pnpm'
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
-      # Changes the working tree only; nothing is committed.
+      # Changes the working tree only; nothing is committed. Outside the
+      # lockfile on purpose, but still under `minimumReleaseAge`, and the job
+      # has no secrets and read-only permissions.
       - name: Install React 18.0.0 and Preact 10
-        run: |
+        run: | # zizmor: ignore[adhoc-packages]
           pnpm add --save-dev react@18.0.0 react-dom@18.0.0 preact@^10
           node -e 'for (const [p, v] of [["react", "18.0.0"], ["react-dom", "18.0.0"], ["preact", "10."]]) { const got = require(`${p}/package.json`).version; console.log(`${p} ${got}`); if (!got.startsWith(v)) throw new Error(`${p} is ${got}, expected ${v}`); }'
       - name: Test
diff --git a/.github/zizmor.yml b/.github/zizmor.yml
new file mode 100644
index 0000000..5eca042
--- /dev/null
+++ b/.github/zizmor.yml
@@ -0,0 +1,12 @@
+# zizmor (https://docs.zizmor.sh) audits GitHub Actions workflows: `pnpm lint:gha`
+# locally, and the lint job in ci.yml.
+rules:
+  unpinned-uses:
+    config:
+      # GitHub's own actions use version tags. Third-party actions are pinned
+      # to a commit SHA, with the version in a trailing comment. Refresh them
+      # with pinact (see CONTRIBUTING.md).
+      policies:
+        actions/*: ref-pin
+        github/*: ref-pin
+        '*': hash-pin
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index a2b8343..c9857e4 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -119,6 +119,33 @@ $ pnpm run lint:fix
 
 `index.js` stays ES5 in 3.x, so don't apply ES2015+ fixes (`let`/`const`, arrow functions) to it.
 
+### GitHub Actions
+
+Workflows reference GitHub's own actions (`actions/*`, `github/*`) by major
+version tag, such as `actions/checkout@v7`. Third-party actions are pinned to a
+commit SHA with the version in a comment, such as
+`pnpm/action-setup@<sha> # v6.1.0`, because a tag can be moved to different
+code. [zizmor](https://docs.zizmor.sh) enforces this (`.github/zizmor.yml`)
+along with its other security checks.
+
+CI runs zizmor in the `lint-gha` job, at the `version` pinned there.
+`pnpm lint:gha` (part of `pnpm check`) runs it locally if it's installed, and
+skips otherwise. Newer zizmor releases add audits, so a different version
+locally can report findings that CI doesn't. To match CI, install that version,
+e.g. `uv tool install zizmor==1.31.1` (or `brew install zizmor` for the
+latest). Set `GH_TOKEN` to include zizmor's online audits.
+
+To update actions:
+
+- Third-party actions: run `pinact run --update`
+  ([pinact](https://github.com/suzuki-shunsuke/pinact), `brew install pinact`).
+  It updates SHAs and their version comments, skipping versions published in
+  the last week (`.github/pinact.yml`). Review each action's release notes.
+- GitHub's actions: check each one's releases page for a new major and read its
+  breaking changes.
+- zizmor itself: bump `version` on the zizmor step in
+  `.github/workflows/ci.yml`.
+
 ### Published package
 
 `pnpm pack:check` (part of `pnpm check`) packs the package and checks what it publishes:
diff --git a/package.json b/package.json
index 4bc0845..19412df 100644
--- a/package.json
+++ b/package.json
@@ -5,12 +5,13 @@
   "main": "index.js",
   "typings": "index.d.ts",
   "scripts": {
-    "check": "pnpm format && pnpm lint && pnpm types:check && pnpm test && pnpm pack:check",
+    "check": "pnpm format && pnpm lint && pnpm lint:gha && pnpm types:check && pnpm test && pnpm pack:check",
     "benchmark": "node benchmark",
     "format": "oxfmt --check",
     "format:fix": "oxfmt",
     "lint": "oxlint --report-unused-disable-directives-severity=error",
     "lint:fix": "oxlint --report-unused-disable-directives-severity=error --fix",
+    "lint:gha": "node scripts/lint-gha.mjs",
     "types:check": "node scripts/types-check.mjs",
     "test": "vitest run",
     "changeset": "changeset",
diff --git a/scripts/lint-gha.mjs b/scripts/lint-gha.mjs
new file mode 100644
index 0000000..61e901f
--- /dev/null
+++ b/scripts/lint-gha.mjs
@@ -0,0 +1,15 @@
+// Runs zizmor (https://docs.zizmor.sh) on .github/ with .github/zizmor.yml.
+// zizmor isn't a Node tool, so this skips with a hint if it isn't installed;
+// CI runs it in the lint-gha job (ci.yml).
+import { spawnSync } from 'node:child_process';
+
+const result = spawnSync('zizmor', ['.'], { stdio: 'inherit' });
+
+if (result.error?.code === 'ENOENT') {
+  process.stdout.write(
+    "zizmor is not installed, skipping (CI runs it). To match CI's version, see CONTRIBUTING.md.\n"
+  );
+  process.exit(0);
+}
+if (result.error) throw result.error;
+process.exit(result.status ?? 1);