diff --git a/.changeset/six-pants-melt.md b/.changeset/six-pants-melt.md
new file mode 100644
index 000000000000..9491b8e50c1d
--- /dev/null
+++ b/.changeset/six-pants-melt.md
@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix `write_types` on windows
diff --git a/packages/kit/src/core/sync/write_types/index.js b/packages/kit/src/core/sync/write_types/index.js
index b925be7735b3..b9f1245e4f07 100644
--- a/packages/kit/src/core/sync/write_types/index.js
+++ b/packages/kit/src/core/sync/write_types/index.js
@@ -68,7 +68,7 @@ export async function write_types(config, manifest_data, file) {
 		return;
 	}
 
-	const id = path.relative(config.kit.files.routes, path.dirname(file));
+	const id = path.posix.relative(config.kit.files.routes, path.dirname(file));
 
 	const route = manifest_data.routes.find((route) => route.id === id);
 	if (!route) return; // this shouldn't ever happen