diff --git a/.changeset/shy-singers-hide.md b/.changeset/shy-singers-hide.md
new file mode 100644
index 00000000..6658f542
--- /dev/null
+++ b/.changeset/shy-singers-hide.md
@@ -0,0 +1,5 @@
+---
+'microbundle': patch
+---
+
+Ensures JS format is not included in CSS filename output
diff --git a/src/index.js b/src/index.js
index 3a5ea8d9..30244f05 100644
--- a/src/index.js
+++ b/src/index.js
@@ -457,7 +457,12 @@ function createConfig(options, entry, format, writeMeta) {
 						modules: cssModulesConfig(options),
 						// only write out CSS for the first bundle (avoids pointless extra files):
 						inject: false,
-						extract: options.css !== 'inline',
+						extract:
+							options.css !== 'inline' &&
+							options.output.replace(
+								/(\.(umd|cjs|es|m))?\.(mjs|[tj]sx?)$/,
+								'.css',
+							),
 						minimize: options.compress,
 						sourceMap: options.sourcemap && options.css !== 'inline',
 					}),