diff --git a/.changeset/tricky-meals-perform.md b/.changeset/tricky-meals-perform.md
new file mode 100644
index 000000000000..029af8321852
--- /dev/null
+++ b/.changeset/tricky-meals-perform.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: build service workers in IIFE format
diff --git a/packages/kit/src/exports/vite/build/build_service_worker.js b/packages/kit/src/exports/vite/build/build_service_worker.js
index 919f6be6646e..8bb292dc68c6 100644
--- a/packages/kit/src/exports/vite/build/build_service_worker.js
+++ b/packages/kit/src/exports/vite/build/build_service_worker.js
@@ -70,6 +70,9 @@ export async function build_service_worker(
 					'service-worker': service_worker_entry_file
 				},
 				output: {
+					// default 'es' format would be nicer
+					// iife is workaround for https://github.com/vitejs/vite/issues/15379
+					format: 'iife',
 					entryFileNames: '[name].js',
 					assetFileNames: `${kit.appDir}/immutable/assets/[name].[hash][extname]`,
 					inlineDynamicImports: true