diff --git a/.changeset/chatty-parrots-compare.md b/.changeset/chatty-parrots-compare.md
new file mode 100644
index 000000000000..f273c4ed1817
--- /dev/null
+++ b/.changeset/chatty-parrots-compare.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/sitemap': patch
+---
+
+Fix sitemap generation with a base path
diff --git a/packages/integrations/sitemap/src/index.ts b/packages/integrations/sitemap/src/index.ts
index 99448b9a1349..e6e45ddd1dd6 100644
--- a/packages/integrations/sitemap/src/index.ts
+++ b/packages/integrations/sitemap/src/index.ts
@@ -79,6 +79,8 @@ const createPlugin = (options?: SitemapOptions): AstroIntegration => {
 					}
 
 					let pageUrls = pages.map((p) => {
+						if (p.pathname !== '' && !finalSiteUrl.pathname.endsWith('/'))
+							finalSiteUrl.pathname += '/';
 						const path = finalSiteUrl.pathname + p.pathname;
 						return new URL(path, finalSiteUrl).href;
 					});
diff --git a/packages/integrations/sitemap/test/trailing-slash.test.js b/packages/integrations/sitemap/test/trailing-slash.test.js
index b5b7dd6c15a3..a393fb9f12f5 100644
--- a/packages/integrations/sitemap/test/trailing-slash.test.js
+++ b/packages/integrations/sitemap/test/trailing-slash.test.js
@@ -59,6 +59,22 @@ describe('Trailing slash', () => {
 			const urls = data.urlset.url;
 			expect(urls[0].loc[0]).to.equal('http://example.com/one');
 		});
+		describe('with base path', () => {
+			before(async () => {
+				fixture = await loadFixture({
+					root: './fixtures/trailing-slash/',
+					trailingSlash: 'never',
+					base: '/base',
+				});
+				await fixture.build();
+			});
+
+			it('URLs do not end with trailing slash', async () => {
+				const data = await readXML(fixture.readFile('/sitemap-0.xml'));
+				const urls = data.urlset.url;
+				expect(urls[0].loc[0]).to.equal('http://example.com/base/one');
+			});
+		});
 	});
 
 	describe('trailingSlash: always', () => {
@@ -75,5 +91,21 @@ describe('Trailing slash', () => {
 			const urls = data.urlset.url;
 			expect(urls[0].loc[0]).to.equal('http://example.com/one/');
 		});
+		describe('with base path', () => {
+			before(async () => {
+				fixture = await loadFixture({
+					root: './fixtures/trailing-slash/',
+					trailingSlash: 'always',
+					base: '/base',
+				});
+				await fixture.build();
+			});
+
+			it('URLs end with trailing slash', async () => {
+				const data = await readXML(fixture.readFile('/sitemap-0.xml'));
+				const urls = data.urlset.url;
+				expect(urls[0].loc[0]).to.equal('http://example.com/base/one/');
+			});
+		});
 	});
 });