diff --git a/src/esbuild/postcss.ts b/src/esbuild/postcss.ts
index 49ec0383..3c2075bc 100644
--- a/src/esbuild/postcss.ts
+++ b/src/esbuild/postcss.ts
@@ -2,7 +2,7 @@ import fs from 'fs'
 import path from 'path'
 import { Loader, Plugin, transform } from 'esbuild'
 import { getPostcss } from '../utils'
-import type { Result } from 'postcss-load-config';
+import type { Result } from 'postcss-load-config'
 
 export const postcssPlugin = ({
   css,
@@ -10,7 +10,7 @@ export const postcssPlugin = ({
   cssLoader,
 }: {
   css?: Map<string, string>
-  inject?: boolean
+  inject?: boolean | ((css: string, fileId: string) => string)
   cssLoader?: Loader
 }): Plugin => {
   return {
@@ -28,7 +28,7 @@ export const postcssPlugin = ({
 
         try {
           const result = await loadConfig({}, process.cwd())
-          configCache =  result
+          configCache = result
           return result
         } catch (error: any) {
           if (error.message.includes('No PostCSS Config found in')) {
@@ -123,9 +123,12 @@ export const postcssPlugin = ({
             })
           ).code
 
-          contents = `import styleInject from '#style-inject';styleInject(${JSON.stringify(
-            contents
-          )})`
+          contents =
+            typeof inject === 'function'
+              ? inject(JSON.stringify(contents), args.path)
+              : `import styleInject from '#style-inject';styleInject(${JSON.stringify(
+                  contents
+                )})`
 
           return {
             contents,
diff --git a/src/options.ts b/src/options.ts
index 067f4444..fc3c2afa 100644
--- a/src/options.ts
+++ b/src/options.ts
@@ -197,7 +197,7 @@ export type Options = {
    * Inject CSS as style tags to document head
    * @default {false}
    */
-  injectStyle?: boolean
+  injectStyle?: boolean | ((css: string, fileId: string) => string)
   /**
    * Inject cjs and esm shims if needed
    * @default false
diff --git a/test/index.test.ts b/test/index.test.ts
index f2a740ac..9e7c71e8 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -1245,3 +1245,25 @@ test(`should generate export {} when there are no exports in source file`, async
   expect(outFiles).toEqual(['input.d.ts', 'input.mjs'])
   expect(await getFileContent('dist/input.d.ts')).toContain('export { }')
 })
+
+test('custom inject style function', async () => {
+  const { outFiles, getFileContent } = await run(
+    getTestName(),
+    {
+      'input.ts': `import './style.css'`,
+      'style.css': `.hello { color: red }`,
+      'tsup.config.ts': `
+        export default {
+          entry: ['src/input.ts'],
+          minify: true,
+          format: ['esm', 'cjs'],
+          injectStyle: (css) => {
+            return "__custom_inject_style__(" + css +")";
+          }
+        }`,
+    },
+  )
+  expect(outFiles).toEqual(['input.js', 'input.mjs'])
+  expect(await getFileContent('dist/input.mjs')).toContain('__custom_inject_style__(`.hello{color:red}\n`)')
+  expect(await getFileContent('dist/input.js')).toContain('__custom_inject_style__(`.hello{color:red}\n`)')
+})
\ No newline at end of file