diff --git a/.changeset/fluffy-cups-invent.md b/.changeset/fluffy-cups-invent.md
new file mode 100644
index 000000000000..0ea588ef1e87
--- /dev/null
+++ b/.changeset/fluffy-cups-invent.md
@@ -0,0 +1,5 @@
+---
+"@astrojs/sitemap": patch
+---
+
+Fixes an issue where the base path is missing in `sitemap-index.xml`.
diff --git a/packages/integrations/sitemap/src/index.ts b/packages/integrations/sitemap/src/index.ts
index 9b1ea5ddb8fb..b0548d8f114c 100644
--- a/packages/integrations/sitemap/src/index.ts
+++ b/packages/integrations/sitemap/src/index.ts
@@ -170,6 +170,7 @@ const createPlugin = (options?: SitemapOptions): AstroIntegration => {
 					await simpleSitemapAndIndex({
 						hostname: finalSiteUrl.href,
 						destinationDir: destDir,
+						publicBasePath: config.base,
 						sourceData: urlData,
 						limit: entryLimit,
 						gzip: false,
diff --git a/packages/integrations/sitemap/test/base-path.test.js b/packages/integrations/sitemap/test/base-path.test.js
index 0417e4e2bf35..df3cadcc4efc 100644
--- a/packages/integrations/sitemap/test/base-path.test.js
+++ b/packages/integrations/sitemap/test/base-path.test.js
@@ -16,9 +16,15 @@ describe('URLs with base path', () => {
 		});
 
 		it('Base path is concatenated correctly', async () => {
-			const data = await readXML(fixture.readFile('/client/sitemap-0.xml'));
-			const urls = data.urlset.url;
-			assert.equal(urls[0].loc[0], 'http://example.com/base/one/');
+			const [sitemapZero, sitemapIndex] = await Promise.all([
+				readXML(fixture.readFile('/client/sitemap-0.xml')),
+				readXML(fixture.readFile('/client/sitemap-index.xml')),
+			]);
+			assert.equal(sitemapZero.urlset.url[0].loc[0], 'http://example.com/base/one/');
+			assert.equal(
+				sitemapIndex.sitemapindex.sitemap[0].loc[0],
+				'http://example.com/base/sitemap-0.xml'
+			);
 		});
 	});
 
@@ -32,9 +38,15 @@ describe('URLs with base path', () => {
 		});
 
 		it('Base path is concatenated correctly', async () => {
-			const data = await readXML(fixture.readFile('/sitemap-0.xml'));
-			const urls = data.urlset.url;
-			assert.equal(urls[0].loc[0], 'http://example.com/base/123/');
+			const [sitemapZero, sitemapIndex] = await Promise.all([
+				readXML(fixture.readFile('/sitemap-0.xml')),
+				readXML(fixture.readFile('/sitemap-index.xml')),
+			]);
+			assert.equal(sitemapZero.urlset.url[0].loc[0], 'http://example.com/base/123/');
+			assert.equal(
+				sitemapIndex.sitemapindex.sitemap[0].loc[0],
+				'http://example.com/base/sitemap-0.xml'
+			);
 		});
 	});
 });