diff --git a/index.d.ts b/index.d.ts
index 380e404..53be28a 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -52,10 +52,11 @@ export interface InstrumentationConfig {
      */
     functionQuery: FunctionQuery;
     /**
-     * The name of a custom transform registered via `addTransform`.
-     * When set, takes precedence over `functionQuery.kind`.
+     * A custom transform to apply. When a string, it must match the name of a
+     * transform registered via `addTransform`. When a function, it is called
+     * directly. Either form takes precedence over `functionQuery.kind`.
      */
-    transform?: string;
+    transform?: string | CustomTransform;
 }
 
 /**
diff --git a/lib/transformer.js b/lib/transformer.js
index 76d6c4d..2813958 100644
--- a/lib/transformer.js
+++ b/lib/transformer.js
@@ -164,7 +164,9 @@ class Transformer {
    * @param {...unknown} args - `(node, parent, ancestry)` from esquery traverse.
    */
   #visit (state, ...args) {
-    const transform = this.#customTransforms[state.operator] ?? transforms[state.operator]
+    const transform = typeof state.transform === 'function'
+      ? state.transform
+      : (this.#customTransforms[state.operator] ?? transforms[state.operator])
     const { index = 0 } = state.functionQuery
     const [node] = args
     const type = node.init?.type || node.type
@@ -196,7 +198,7 @@ class Transformer {
    * @returns {string} Operator name, e.g. `'tracePromise'`.
    */
   #getOperator ({ transform, functionQuery: { kind } }) {
-    if (transform) return transform
+    if (typeof transform === 'string') return transform
 
     switch (kind) {
       case 'Async': return 'tracePromise'
diff --git a/tests/custom_transform_fn_cjs/mod.js b/tests/custom_transform_fn_cjs/mod.js
new file mode 100644
index 0000000..4a6b8a9
--- /dev/null
+++ b/tests/custom_transform_fn_cjs/mod.js
@@ -0,0 +1,5 @@
+function fetch (url) {
+  return 42
+}
+
+module.exports = { fetch }
diff --git a/tests/custom_transform_fn_cjs/test.js b/tests/custom_transform_fn_cjs/test.js
new file mode 100644
index 0000000..0794b22
--- /dev/null
+++ b/tests/custom_transform_fn_cjs/test.js
@@ -0,0 +1,5 @@
+const { fetch } = require('./instrumented.js')
+const assert = require('node:assert')
+
+fetch('https://example.com')
+assert.strictEqual(global.__customCalled, true)
diff --git a/tests/tests.test.mjs b/tests/tests.test.mjs
index e2eccdc..402e889 100644
--- a/tests/tests.test.mjs
+++ b/tests/tests.test.mjs
@@ -418,6 +418,35 @@ describe('polyfill_mjs', () => {
   })
 })
 
+describe('custom_transform_fn_cjs', () => {
+  test('applies a custom transform function passed directly in the config', () => {
+    runTest('custom_transform_fn_cjs', [
+      {
+        channelName: 'fetch_custom',
+        module: { name: TEST_MODULE_NAME, versionRange: '>=0.0.1', filePath: TEST_MODULE_PATH },
+        functionQuery: { functionName: 'fetch', kind: 'Sync' },
+        transform (_state, node) {
+          node.body.body.unshift({
+            type: 'ExpressionStatement',
+            expression: {
+              type: 'AssignmentExpression',
+              operator: '=',
+              left: {
+                type: 'MemberExpression',
+                object: { type: 'Identifier', name: 'global' },
+                property: { type: 'Identifier', name: '__customCalled' },
+                computed: false,
+                optional: false,
+              },
+              right: { type: 'Literal', value: true, raw: 'true' },
+            },
+          })
+        },
+      },
+    ])
+  })
+})
+
 describe('custom_transform_cjs', () => {
   test('applies a custom transform registered via addTransform', () => {
     runTest('custom_transform_cjs', [