diff --git a/.github/workflows/tls-cert-check.yml b/.github/workflows/tls-cert-check.yml
new file mode 100644
index 00000000..01e89a02
--- /dev/null
+++ b/.github/workflows/tls-cert-check.yml
@@ -0,0 +1,42 @@
+name: TLS Certificate Expiry Check
+
+on:
+  schedule:
+    - cron: '0 6 * * *'
+  workflow_dispatch:
+    inputs:
+      endpoints:
+        description: 'Comma-separated host[:port] list (overrides repository variable)'
+        required: false
+  pull_request:
+    paths:
+      - 'backend/src/security/**'
+      - 'backend/scripts/check-tls-certs.js'
+      - '.github/workflows/tls-cert-check.yml'
+
+jobs:
+  tls-cert-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      - name: Install backend dependencies
+        run: npm ci --workspace=backend
+
+      - name: Run unit tests
+        run: node --test src/security/*.test.js
+        working-directory: backend
+
+      # Fails (and so notifies maintainers) when any cert expires within 30 days.
+      # Configure endpoints via the TLS_CHECK_ENDPOINTS repository variable.
+      - name: Check certificate expiry
+        if: github.event_name != 'pull_request'
+        run: node scripts/check-tls-certs.js --warn-days=30
+        working-directory: backend
+        env:
+          TLS_CHECK_ENDPOINTS: ${{ github.event.inputs.endpoints || vars.TLS_CHECK_ENDPOINTS }}
diff --git a/backend/package.json b/backend/package.json
index b44d1840..80d1e839 100644
--- a/backend/package.json
+++ b/backend/package.json
@@ -19,7 +19,8 @@
     "docs:build": "node scripts/build-redoc.js",
     "openapi:lint": "redocly lint openapi.yaml",
     "security:xss": "node scripts/security-audit.js",
-    "security-headers-check": "curl -s -I ${CHECK_URL:-http://localhost:3001/health} | grep -iE '(x-content-type|x-frame|strict-transport|content-security|referrer|permissions)'"
+    "security-headers-check": "curl -s -I ${CHECK_URL:-http://localhost:3001/health} | grep -iE '(x-content-type|x-frame|strict-transport|content-security|referrer|permissions)'",
+    "security:tls": "node scripts/check-tls-certs.js"
   },
   "dependencies": {
     "@aws-sdk/client-s3": "^3.1057.0",
diff --git a/backend/scripts/check-tls-certs.js b/backend/scripts/check-tls-certs.js
new file mode 100644
index 00000000..23ccf7f1
--- /dev/null
+++ b/backend/scripts/check-tls-certs.js
@@ -0,0 +1,33 @@
+#!/usr/bin/env node
+// Usage: node scripts/check-tls-certs.js [--warn-days=30] host[:port] ...
+// Endpoints may also come from TLS_CHECK_ENDPOINTS (comma separated).
+// Exits 1 if any certificate expires within the warning window, is expired, or cannot be checked.
+import { checkEndpoints, DEFAULT_WARN_DAYS } from '../src/security/tlsCertCheck.js';
+
+const args = process.argv.slice(2);
+let warnDays = Number(process.env.TLS_WARN_DAYS ?? DEFAULT_WARN_DAYS);
+const endpoints = (process.env.TLS_CHECK_ENDPOINTS || '').split(',').map((s) => s.trim()).filter(Boolean);
+for (const arg of args) {
+  if (arg.startsWith('--warn-days=')) warnDays = Number(arg.split('=')[1]);
+  else endpoints.push(arg);
+}
+
+if (!Number.isFinite(warnDays) || warnDays < 0 || endpoints.length === 0) {
+  console.error('Usage: check-tls-certs.js [--warn-days=30] host[:port] ... (or set TLS_CHECK_ENDPOINTS)');
+  process.exit(2);
+}
+
+const results = await checkEndpoints(endpoints, { warnDays });
+let failed = false;
+for (const r of results) {
+  if (r.status === 'ok') {
+    console.log(`OK      ${r.endpoint}: ${r.daysRemaining} days remaining`);
+  } else if (r.status === 'error') {
+    failed = true;
+    console.error(`::error::TLS check failed for ${r.endpoint}: ${r.error}`);
+  } else {
+    failed = true;
+    console.error(`::error::TLS certificate for ${r.endpoint} ${r.status === 'expired' ? 'has expired' : `expires in ${r.daysRemaining} days`} (threshold ${warnDays})`);
+  }
+}
+process.exit(failed ? 1 : 0);
diff --git a/backend/src/security/tlsCertCheck.js b/backend/src/security/tlsCertCheck.js
new file mode 100644
index 00000000..224a944d
--- /dev/null
+++ b/backend/src/security/tlsCertCheck.js
@@ -0,0 +1,78 @@
+import tls from 'node:tls';
+
+export const DEFAULT_WARN_DAYS = 30;
+const DAY_MS = 24 * 60 * 60 * 1000;
+
+/**
+ * Parse a host spec ("example.com", "example.com:8443" or an https URL)
+ * into { host, port }. Throws on invalid input.
+ */
+export function parseEndpoint(spec) {
+  if (typeof spec !== 'string' || !spec.trim()) {
+    throw new TypeError('Endpoint must be a non-empty string');
+  }
+  const raw = spec.trim();
+  const url = new URL(raw.includes('://') ? raw : `https://${raw}`);
+  if (!url.hostname) throw new TypeError(`Invalid endpoint: ${spec}`);
+  return { host: url.hostname, port: url.port ? Number(url.port) : 443 };
+}
+
+/** Whole days between `now` and `validTo` (negative when expired). */
+export function daysUntil(validTo, now = new Date()) {
+  const end = new Date(validTo).getTime();
+  if (Number.isNaN(end)) throw new TypeError(`Invalid certificate date: ${validTo}`);
+  return Math.floor((end - now.getTime()) / DAY_MS);
+}
+
+/** Classify remaining days as ok | warning | expired. */
+export function classify(daysRemaining, warnDays = DEFAULT_WARN_DAYS) {
+  if (daysRemaining < 0) return 'expired';
+  if (daysRemaining <= warnDays) return 'warning';
+  return 'ok';
+}
+
+/** Fetch the peer certificate's expiry from a live TLS endpoint. */
+export function fetchCertExpiry({ host, port = 443 }, { timeoutMs = 10000, connect = tls.connect } = {}) {
+  return new Promise((resolve, reject) => {
+    const socket = connect({ host, port, servername: host, rejectUnauthorized: false });
+    const fail = (err) => {
+      socket.destroy();
+      reject(err);
+    };
+    socket.setTimeout(timeoutMs, () => fail(new Error(`Timed out connecting to ${host}:${port}`)));
+    socket.once('error', fail);
+    socket.once('secureConnect', () => {
+      const cert = socket.getPeerCertificate();
+      socket.end();
+      if (!cert || !cert.valid_to) {
+        reject(new Error(`No certificate returned by ${host}:${port}`));
+        return;
+      }
+      resolve({ validTo: cert.valid_to, subject: cert.subject?.CN, issuer: cert.issuer?.CN });
+    });
+  });
+}
+
+/**
+ * Check a list of endpoints. Never throws for a single endpoint failure;
+ * failures are reported with status "error".
+ */
+export async function checkEndpoints(specs, { warnDays = DEFAULT_WARN_DAYS, now = new Date(), fetchExpiry = fetchCertExpiry } = {}) {
+  return Promise.all(
+    specs.map(async (spec) => {
+      try {
+        const endpoint = parseEndpoint(spec);
+        const info = await fetchExpiry(endpoint);
+        const daysRemaining = daysUntil(info.validTo, now);
+        return {
+          endpoint: spec,
+          ...info,
+          daysRemaining,
+          status: classify(daysRemaining, warnDays),
+        };
+      } catch (err) {
+        return { endpoint: spec, status: 'error', error: err.message };
+      }
+    }),
+  );
+}
diff --git a/backend/src/security/tlsCertCheck.test.js b/backend/src/security/tlsCertCheck.test.js
new file mode 100644
index 00000000..f5148d02
--- /dev/null
+++ b/backend/src/security/tlsCertCheck.test.js
@@ -0,0 +1,42 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { parseEndpoint, daysUntil, classify, checkEndpoints } from './tlsCertCheck.js';
+
+const now = new Date('2026-01-01T00:00:00Z');
+
+test('parseEndpoint handles host, port and url forms', () => {
+  assert.deepEqual(parseEndpoint('example.com'), { host: 'example.com', port: 443 });
+  assert.deepEqual(parseEndpoint('example.com:8443'), { host: 'example.com', port: 8443 });
+  assert.deepEqual(parseEndpoint('https://api.example.com/health'), { host: 'api.example.com', port: 443 });
+  assert.throws(() => parseEndpoint(''), TypeError);
+  assert.throws(() => parseEndpoint(42), TypeError);
+});
+
+test('daysUntil computes whole days and rejects bad dates', () => {
+  assert.equal(daysUntil('2026-01-31T00:00:00Z', now), 30);
+  assert.equal(daysUntil('2025-12-30T00:00:00Z', now), -2);
+  assert.throws(() => daysUntil('garbage', now), TypeError);
+});
+
+test('classify boundaries', () => {
+  assert.equal(classify(-1), 'expired');
+  assert.equal(classify(0), 'warning');
+  assert.equal(classify(30), 'warning');
+  assert.equal(classify(31), 'ok');
+  assert.equal(classify(10, 5), 'ok');
+});
+
+test('checkEndpoints reports ok, warning, expired and error', async () => {
+  const dates = {
+    'ok.example': '2026-06-01T00:00:00Z',
+    'soon.example': '2026-01-15T00:00:00Z',
+    'old.example': '2025-12-01T00:00:00Z',
+  };
+  const fetchExpiry = async ({ host }) => {
+    if (!dates[host]) throw new Error('boom');
+    return { validTo: dates[host] };
+  };
+  const res = await checkEndpoints(['ok.example', 'soon.example', 'old.example', 'bad.example'], { now, fetchExpiry });
+  assert.deepEqual(res.map((r) => r.status), ['ok', 'warning', 'expired', 'error']);
+  assert.equal(res[3].error, 'boom');
+});