diff --git a/.github/workflows/validate-new-gap-number.yml b/.github/workflows/validate-new-gap-number.yml
new file mode 100644
index 0000000..1806e79
--- /dev/null
+++ b/.github/workflows/validate-new-gap-number.yml
@@ -0,0 +1,28 @@
+name: Validate new GAP number
+
+on:
+  pull_request:
+    branches: [main]
+
+jobs:
+  validate-gap-number:
+    name: Validate new GAP id matches PR number
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v6
+        with:
+          node-version: "24"
+          cache: "npm"
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Validate new GAP id matches PR number
+        run: node scripts/validate-new-gap-number.js --pr-number ${{ github.event.pull_request.number }}
diff --git a/scripts/validate-new-gap-number.js b/scripts/validate-new-gap-number.js
new file mode 100755
index 0000000..331ba62
--- /dev/null
+++ b/scripts/validate-new-gap-number.js
@@ -0,0 +1,69 @@
+#!/usr/bin/env node
+
+/**
+ * Validates that newly added GAP directories and their metadata.yml `id` field
+ * both match the PR number, per CONTRIBUTING.md.
+ *
+ * Usage: node scripts/validate-new-gap-number.js --pr-number 123
+ */
+
+import { readFileSync } from "node:fs";
+import { execSync } from "node:child_process";
+import { dirname, basename } from "node:path";
+import { parseArgs } from "node:util";
+import { parse as parseYaml } from "yaml";
+
+const { values } = parseArgs({
+  options: {
+    "pr-number": { type: "string" },
+  },
+  strict: true,
+});
+
+const prNumber = values["pr-number"];
+
+const newMetadataFiles = execSync(
+  "git diff --name-only --diff-filter=A origin/main...HEAD -- 'gaps/*/metadata.yml'",
+  { encoding: "utf8" },
+)
+  .trim()
+  .split("\n")
+  .filter(Boolean);
+
+if (newMetadataFiles.length === 0) {
+  console.log("No new metadata.yml files added in this PR.");
+  process.exit(0);
+}
+
+let failed = false;
+
+for (const file of newMetadataFiles) {
+  const gapDir = basename(dirname(file));
+  const expectedDir = `GAP-${prNumber}`;
+
+  if (gapDir !== expectedDir) {
+    console.error(
+      `::error file=${file}::GAP directory is ${gapDir}, but must be ${expectedDir} to match the PR number. See CONTRIBUTING.md.`,
+    );
+    failed = true;
+  } else {
+    console.log(`✓ ${file}: directory ${gapDir} matches PR #${prNumber}`);
+  }
+
+  const content = readFileSync(file, "utf8");
+  const metadata = parseYaml(content);
+  const id = String(metadata.id);
+
+  if (id !== prNumber) {
+    console.error(
+      `::error file=${file}::metadata.yml 'id' field is ${id}, but must match the PR number (${prNumber}). See CONTRIBUTING.md.`,
+    );
+    failed = true;
+  } else {
+    console.log(`✓ ${file}: id ${id} matches PR #${prNumber}`);
+  }
+}
+
+if (failed) {
+  process.exit(1);
+}