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An XML sitemap is valid when it is well-formed XML rooted at <urlset> or <sitemapindex>, every entry carries exactly one absolute <loc>, and the file stays inside the sitemap protocol's limits of 50,000 URLs and 50MB uncompressed. Check your sitemap.xml is well-formed and valid before you submit it — paste or upload the file and see a PASS/FAIL checklist. Everything runs in your browser.
Paste the contents of your sitemap.xml into the box, or use Load a sitemap file to read one from your computer — the file's text drops straight into the box. The validator runs as you type: it parses the XML, confirms the root is a <urlset> or <sitemapindex>, and checks that every <loc> is an absolute http(s) URL, that any <lastmod>, <changefreq> and <priority> values are valid, and that the file stays within the 50,000-entry and 50MB limits. A big PASS or FAIL badge, a URL count, and a checklist show exactly what passed, with any errors and warnings listed underneath.
This validates structure and syntax only, locally in your browser — it does not fetch or crawl the URLs, and it does not check that the pages exist or are indexable. Need to build a sitemap first? Use the sitemap generator. Once it is live, check that the pages it lists are not blocked with the robots.txt tester, and write the Sitemap: line itself with the robots.txt generator.