diff --git a/docs/README.md b/docs/README.md
index 7a86923..e606c83 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -283,6 +283,27 @@ Passing `--metafile` flag to tell esbuild to produce some metadata about the bui
 
 The file outputs as `metafile-{format}.json`, e.g. `tsup --format cjs,esm` will generate `metafile-cjs.json` and `metafile-esm.json`.
 
+### Custom esbuild plugin and options
+
+Use `esbuildPlugins` and `esbuildOptions` respectively in `tsup.config.ts`:
+
+```ts
+import { Options } from 'tsup'
+
+export const tsup: Options = {
+  esbuildPlugins: [YourPlugin],
+  esbuildOptions(options, context) {
+    options.define.foo = '"bar"'
+  },
+}
+```
+
+The `context` argument for `esbuildOptions`:
+
+- `context.format`: `cjs`, `esm`, `iife`
+
+See all options [here](https://esbuild.github.io/api/#build-api), and [how to write an esbuild plugin](https://esbuild.github.io/plugins/#using-plugins).
+
 ---
 
 For more details:
diff --git a/src/index.ts b/src/index.ts
index ee5bbff..457a251 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,5 +1,5 @@
 import fs from 'fs'
-import path, { dirname, join, extname, basename } from 'path'
+import path, { dirname, join, extname } from 'path'
 import { Worker } from 'worker_threads'
 import type { InputOption } from 'rollup'
 import { transform as transformToEs5 } from 'buble'
@@ -90,6 +90,7 @@ export type Options = {
    */
   clean?: boolean | string[]
   esbuildPlugins?: EsbuildPlugin[]
+  esbuildOptions?: (options: BuildOptions, context: { format: Format }) => void
   /**
    * Supress non-error logs (excluding "onSuccess" process output)
    */
@@ -191,6 +192,14 @@ export async function runEsbuild(
       sourcemap: options.sourcemap,
       target: options.target === 'es5' ? 'es2016' : options.target,
       plugins: [
+        {
+          name: 'modify-options',
+          setup(build) {
+            if (options.esbuildOptions) {
+              options.esbuildOptions(build.initialOptions, { format })
+            }
+          },
+        },
         // esbuild's `external` option doesn't support RegExp
         // So here we use a custom plugin to implement it
         externalPlugin({