diff --git a/src/esbuild/postcss.ts b/src/esbuild/postcss.ts
index 1c754022..ec6fb599 100644
--- a/src/esbuild/postcss.ts
+++ b/src/esbuild/postcss.ts
@@ -9,7 +9,7 @@ export const postcssPlugin = ({
   cssLoader,
 }: {
   css?: Map<string, string>
-  inject?: boolean | ((css: string, fileId: string) => string)
+  inject?: boolean | ((css: string, fileId: string) => Promise<string>)
   cssLoader?: Loader
 }): Plugin => {
   return {
@@ -124,7 +124,7 @@ export const postcssPlugin = ({
 
           contents =
             typeof inject === 'function'
-              ? inject(JSON.stringify(contents), args.path)
+              ? await inject(JSON.stringify(contents), args.path)
               : `import styleInject from '#style-inject';styleInject(${JSON.stringify(
                   contents,
                 )})`
diff --git a/src/options.ts b/src/options.ts
index 68c8d3ff..ed09cb5e 100644
--- a/src/options.ts
+++ b/src/options.ts
@@ -210,7 +210,7 @@ export type Options = {
    * Inject CSS as style tags to document head
    * @default {false}
    */
-  injectStyle?: boolean | ((css: string, fileId: string) => string)
+  injectStyle?: boolean | ((css: string, fileId: string) => Promise<string>)
   /**
    * Inject cjs and esm shims if needed
    * @default false