diff --git a/.changeset/hot-brooms-sleep.md b/.changeset/hot-brooms-sleep.md
new file mode 100644
index 000000000000..53f85d0ee5dd
--- /dev/null
+++ b/.changeset/hot-brooms-sleep.md
@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Fix TypeError when a GET form uses a relative `formaction` attribute with ClientRouter enabled
diff --git a/packages/astro/components/ClientRouter.astro b/packages/astro/components/ClientRouter.astro
index 57685589c4d6..511765ef30de 100644
--- a/packages/astro/components/ClientRouter.astro
+++ b/packages/astro/components/ClientRouter.astro
@@ -136,7 +136,7 @@ const { fallback = 'animate' } = Astro.props;
 			const options: Options = { sourceElement: submitter ?? form };
 			if (method === 'get') {
 				const params = new URLSearchParams(formData as any);
-				const url = new URL(action);
+				const url = new URL(action, location.href);
 				url.search = params.toString();
 				action = url.toString();
 			} else {