diff --git a/astro.config.mjs b/astro.config.mjs
index 015b71f0..b5eae161 100644
--- a/astro.config.mjs
+++ b/astro.config.mjs
@@ -1,17 +1,79 @@
 // @ts-check
+import { readFileSync, readdirSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
 import { defineConfig } from 'astro/config';
 import mdx from '@astrojs/mdx';
 import sitemap from '@astrojs/sitemap';
 import tailwindcss from '@tailwindcss/vite';
 
+const SITE = 'https://specification.website';
+
+/**
+ * Build a map of canonical URL -> last-modified date, derived from the `updated`
+ * front matter of every spec entry. This is the same source of truth the RSS feed
+ * and the spec pages use, so the sitemap's <lastmod> stays in sync automatically.
+ */
+function buildLastmodMap() {
+  const specDir = fileURLToPath(new URL('./src/content/spec', import.meta.url));
+  /** @type {Map<string, string>} per-URL lastmod (YYYY-MM-DD) */
+  const urls = new Map();
+  /** @type {Map<string, string>} per-category newest lastmod */
+  const byCategory = new Map();
+  let newest = '';
+
+  for (const category of readdirSync(specDir, { withFileTypes: true })) {
+    if (!category.isDirectory()) continue;
+    const catDir = `${specDir}/${category.name}`;
+    for (const file of readdirSync(catDir)) {
+      if (!/\.(md|mdx)$/.test(file)) continue;
+      const raw = readFileSync(`${catDir}/${file}`, 'utf8');
+      const fm = raw.match(/^---\r?\n([\s\S]*?)\r?\n---/);
+      if (!fm) continue;
+      const front = fm[1];
+      if (/^draft:\s*true\s*$/m.test(front)) continue;
+
+      const updated = front.match(/^updated:\s*["']?([0-9]{4}-[0-9]{2}-[0-9]{2})["']?\s*$/m)?.[1];
+      if (!updated) continue;
+      const cat = front.match(/^category:\s*["']?([\w-]+)["']?\s*$/m)?.[1] ?? category.name;
+      const slug =
+        front.match(/^slug:\s*["']?([\w-]+)["']?\s*$/m)?.[1] ?? file.replace(/\.(md|mdx)$/, '');
+
+      urls.set(`${SITE}/spec/${cat}/${slug}/`, updated);
+
+      if (updated > (byCategory.get(cat) ?? '')) byCategory.set(cat, updated);
+      if (updated > newest) newest = updated;
+    }
+  }
+
+  // Category index pages reflect the newest entry they list.
+  for (const [cat, updated] of byCategory) {
+    urls.set(`${SITE}/spec/${cat}/`, updated);
+  }
+  // Surfaces derived from the whole collection track the newest entry overall.
+  if (newest) {
+    for (const path of ['/', '/spec/', '/checklist/']) {
+      urls.set(`${SITE}${path}`, newest);
+    }
+  }
+
+  return urls;
+}
+
+const lastmodByUrl = buildLastmodMap();
+
 // https://astro.build/config
 export default defineConfig({
-  site: 'https://specification.website',
+  site: SITE,
   integrations: [
     mdx(),
     sitemap({
       changefreq: 'weekly',
       priority: 0.7,
+      serialize(item) {
+        const lastmod = lastmodByUrl.get(item.url);
+        if (lastmod) item.lastmod = new Date(`${lastmod}T00:00:00Z`).toISOString();
+        return item;
+      },
     }),
   ],
   vite: {
diff --git a/src/content/spec/seo/xml-sitemaps.md b/src/content/spec/seo/xml-sitemaps.md
index 4750f94c..f5a3b289 100644
--- a/src/content/spec/seo/xml-sitemaps.md
+++ b/src/content/spec/seo/xml-sitemaps.md
@@ -58,6 +58,8 @@ Follow the spec:
 
 Generate sitemaps dynamically from your content source, not by crawling your own site — that way you cannot accidentally include orphaned or redirected URLs.
 
+**This site ships it.** `specification.website` generates [`/sitemap-index.xml`](/sitemap-index.xml) at build time from the content collection, and sets each `<lastmod>` from the entry's `updated` front matter — the same field the [RSS feed](/rss.xml) uses — rather than the build timestamp, so the date only moves when the content actually changes.
+
 ## Common mistakes
 
 - Listing non-canonical URLs (parameters, session IDs, alternate-case paths).