diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index cd6c7639f1d3..c243997baa3e 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -127,6 +127,7 @@ Front matter is defined as a YAML block at the top of a Markdown document that d
 - `feature_image`: Blog posts only. Relative path to a high-resolution hero image (1884×1256) displayed at the top of the blog post page.
 - `meta_title`: If specified, the meta title (for OpenGraph) will use this value instead of the value in the `title` attribute.
 - `redirect_to`: The relative or absolute URL of a permanent redirect.
+- `sitemap_exclude`: Set to `true` to omit the page from the generated `sitemap.xml` without affecting crawling or indexing (unlike `block_external_search_index`, which also adds a `noindex` directive). Use this only for pages whose canonical URL is already declared, with an accurate `lastmod`, in a different sitemap that Google is submitted (for example a page that is a build-time placeholder for a URL actually served and indexed from a different origin).
 - `title`: Required (unless `redirect_to` is set), 60 characters or less. This controls the default value for the `<title>` tag as well at the top level `<h1>` in the document.
 - `title_tag`: If specified, the `<title>` tag on the rendered call will use this value instead of the `title` attribute.
 
diff --git a/content/registry.md b/content/registry.md
index a6ff7f459bc1..e2bc3185c082 100644
--- a/content/registry.md
+++ b/content/registry.md
@@ -1,5 +1,15 @@
 ---
 title: "Pulumi Registry"
+# This file is a build-time placeholder only (see the readme note below); the
+# CloudFront/S3 trick means Google never actually sees the HTML Hugo renders
+# from it. The real https://www.pulumi.com/registry/ page is served by, and
+# already listed with an accurate lastmod in the sitemap of, the Registry app
+# (github.com/pulumi/registry). Without sitemap_exclude, this placeholder's own
+# git-derived lastmod (frozen since the 2023-12-11 commit that introduced it)
+# competes with the Registry sitemap's entry for the same canonical URL, and
+# consistently wins the "oldest URL in the whole sitemap" contest. Do not
+# remove sitemap_exclude without also removing the duplicate from one side.
+sitemap_exclude: true
 readme: |
     This page is intentionally left blank, which may seem strange, but is actually necessary because
     of how the CloudFront CDN is configured to serve https://www.pulumi.com/registry/.
diff --git a/layouts/_default/sitemap.xml b/layouts/_default/sitemap.xml
index 9e01e7995d29..84fa6689b4e8 100644
--- a/layouts/_default/sitemap.xml
+++ b/layouts/_default/sitemap.xml
@@ -1,7 +1,7 @@
 <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
   xmlns:xhtml="http://www.w3.org/1999/xhtml">
   {{ range .Data.Pages }}
-  {{ if not (or .Params.private .Params.block_external_search_index .Params.redirect_to .Params.redirect_url (not (partial "taxonomy-indexable.html" .))) }}
+  {{ if not (or .Params.private .Params.block_external_search_index .Params.redirect_to .Params.redirect_url .Params.sitemap_exclude (not (partial "taxonomy-indexable.html" .))) }}
   {{ $lastmod := .Lastmod }}
   {{ if in (slice "home" "section" "taxonomy" "term") .Kind }}
   {{ range .RegularPagesRecursive }}