diff --git a/.changeset/eighty-shoes-flow.md b/.changeset/eighty-shoes-flow.md
new file mode 100644
index 00000000000..3fc449c1546
--- /dev/null
+++ b/.changeset/eighty-shoes-flow.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/start-plugin-core': patch
+---
+
+fix: use http scheme for sitemap xmlns per sitemaps.org spec
diff --git a/packages/start-plugin-core/src/build-sitemap.ts b/packages/start-plugin-core/src/build-sitemap.ts
index 3eae17ca021..9d2c5c1ecd2 100644
--- a/packages/start-plugin-core/src/build-sitemap.ts
+++ b/packages/start-plugin-core/src/build-sitemap.ts
@@ -202,7 +202,7 @@ export function buildSitemap({
 function createXml(elementName: 'urlset' | 'sitemapindex'): XMLBuilder {
   return create({ version: '1.0', encoding: 'UTF-8' })
     .ele(elementName, {
-      xmlns: 'https://www.sitemaps.org/schemas/sitemap/0.9',
+      xmlns: 'http://www.sitemaps.org/schemas/sitemap/0.9',
       'xmlns:xhtml': 'http://www.w3.org/1999/xhtml',
     })
     .com(`This file was automatically generated by TanStack Start.`)