diff --git a/types/util.promisify/auto.d.ts b/types/util.promisify/auto.d.ts
index 111dd3a6c9..a09dcc48f3 100644
--- a/types/util.promisify/auto.d.ts
+++ b/types/util.promisify/auto.d.ts
@@ -1,3 +1,13 @@
-import shim = require("./shim");
+import polyfill = require('./index');
 
-export {};
+declare module 'util' {
+	namespace promisify {
+		/**
+		 * @deprecated
+		 * Not exposed by native `util.promisify` or supported by browserify's `util.promisify`.
+		 *
+		 * Use `util.promisify.custom` instead.
+		 */
+		const customPromisifyArgs: typeof polyfill.customPromisifyArgs | undefined;
+	}
+}
diff --git a/types/util.promisify/implementation.d.ts b/types/util.promisify/implementation.d.ts
new file mode 100644
index 0000000000..4a1d295ca6
--- /dev/null
+++ b/types/util.promisify/implementation.d.ts
@@ -0,0 +1,59 @@
+import util = require('util');
+
+// tslint:disable-next-line: ban-types
+declare function promisify<TCustom extends Function>(fn: util.CustomPromisify<TCustom>): TCustom;
+
+declare function promisify<TResult>(
+	fn: (callback: (err: any, result: TResult) => void) => void,
+): () => Promise<TResult>;
+declare function promisify(fn: (callback: (err?: any) => void) => void): () => Promise<void>;
+
+declare function promisify<T1, TResult>(
+	fn: (arg1: T1, callback: (err: any, result: TResult) => void) => void,
+): (arg1: T1) => Promise<TResult>;
+declare function promisify<T1>(fn: (arg1: T1, callback: (err?: any) => void) => void): (arg1: T1) => Promise<void>;
+
+declare function promisify<T1, T2, TResult>(
+	fn: (arg1: T1, arg2: T2, callback: (err: any, result: TResult) => void) => void,
+): (arg1: T1, arg2: T2) => Promise<TResult>;
+declare function promisify<T1, T2>(
+	fn: (arg1: T1, arg2: T2, callback: (err?: any) => void) => void,
+): (arg1: T1, arg2: T2) => Promise<void>;
+
+declare function promisify<T1, T2, T3, TResult>(
+	fn: (arg1: T1, arg2: T2, arg3: T3, callback: (err: any, result: TResult) => void) => void,
+): (arg1: T1, arg2: T2, arg3: T3) => Promise<TResult>;
+declare function promisify<T1, T2, T3>(
+	fn: (arg1: T1, arg2: T2, arg3: T3, callback: (err?: any) => void) => void,
+): (arg1: T1, arg2: T2, arg3: T3) => Promise<void>;
+
+declare function promisify<T1, T2, T3, T4, TResult>(
+	fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, callback: (err: any, result: TResult) => void) => void,
+): (arg1: T1, arg2: T2, arg3: T3, arg4: T4) => Promise<TResult>;
+declare function promisify<T1, T2, T3, T4>(
+	fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, callback: (err?: any) => void) => void,
+): (arg1: T1, arg2: T2, arg3: T3, arg4: T4) => Promise<void>;
+
+declare function promisify<T1, T2, T3, T4, T5, TResult>(
+	fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, callback: (err: any, result: TResult) => void) => void,
+): (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5) => Promise<TResult>;
+declare function promisify<T1, T2, T3, T4, T5>(
+	fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, callback: (err?: any) => void) => void,
+): (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5) => Promise<void>;
+
+// tslint:disable-next-line: ban-types
+declare function promisify(fn: Function): Function;
+
+declare namespace promisify {
+	const custom: typeof util.promisify.custom;
+
+	/**
+	 * @deprecated
+	 * Not exposed by native `util.promisify` or supported by browserify's `util.promisify`.
+	 *
+	 * Use `util.promisify.custom` instead.
+	 */
+	const customPromisifyArgs: unique symbol;
+}
+
+export = promisify;
diff --git a/types/util.promisify/index.d.ts b/types/util.promisify/index.d.ts
index dee4bb30a0..64d4877486 100644
--- a/types/util.promisify/index.d.ts
+++ b/types/util.promisify/index.d.ts
@@ -2,40 +2,75 @@
 // Project: https://github.com/ljharb/util.promisify#readme
 // Definitions by: Adam Voss <https://github.com/adamvoss>
 //                 Piotr Roszatycki <https://github.com/dex4er>
+//                 ExE Boss <https://github.com/ExE-Boss>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 /// <reference types="node" />
 
-// tslint:disable:ban-types
-interface CustomPromisify<TCustom extends Function> extends Function {
-    __promisify__: TCustom;
-}
+import util = require('util');
+
+import polyfill = require('./implementation');
+import getUtilPromisify = require('./polyfill');
+import shimUtilPromisify = require('./shim');
+
+// tslint:disable-next-line: ban-types
+declare function promisify<TCustom extends Function>(fn: util.CustomPromisify<TCustom>): TCustom;
+
+declare function promisify<TResult>(
+	fn: (callback: (err: any, result: TResult) => void) => void,
+): () => Promise<TResult>;
+declare function promisify(fn: (callback: (err?: any) => void) => void): () => Promise<void>;
+
+declare function promisify<T1, TResult>(
+	fn: (arg1: T1, callback: (err: any, result: TResult) => void) => void,
+): (arg1: T1) => Promise<TResult>;
+declare function promisify<T1>(fn: (arg1: T1, callback: (err?: any) => void) => void): (arg1: T1) => Promise<void>;
+
+declare function promisify<T1, T2, TResult>(
+	fn: (arg1: T1, arg2: T2, callback: (err: any, result: TResult) => void) => void,
+): (arg1: T1, arg2: T2) => Promise<TResult>;
+declare function promisify<T1, T2>(
+	fn: (arg1: T1, arg2: T2, callback: (err?: any) => void) => void,
+): (arg1: T1, arg2: T2) => Promise<void>;
+
+declare function promisify<T1, T2, T3, TResult>(
+	fn: (arg1: T1, arg2: T2, arg3: T3, callback: (err: any, result: TResult) => void) => void,
+): (arg1: T1, arg2: T2, arg3: T3) => Promise<TResult>;
+declare function promisify<T1, T2, T3>(
+	fn: (arg1: T1, arg2: T2, arg3: T3, callback: (err?: any) => void) => void,
+): (arg1: T1, arg2: T2, arg3: T3) => Promise<void>;
 
-declare function promisify<TCustom extends Function>(fn: CustomPromisify<TCustom>): TCustom;
-declare function promisify<TResult>(fn: (callback: (err: Error | null, result: TResult) => void) => void): () => Promise<TResult>;
-declare function promisify(fn: (callback: (err?: Error | null) => void) => void): () => Promise<void>;
-declare function promisify<T1, TResult>(fn: (arg1: T1, callback: (err: Error | null, result: TResult) => void) => void): (arg1: T1) => Promise<TResult>;
-declare function promisify<T1>(fn: (arg1: T1, callback: (err?: Error | null) => void) => void): (arg1: T1) => Promise<void>;
-declare function promisify<T1, T2, TResult>(fn: (arg1: T1, arg2: T2, callback: (err: Error | null, result: TResult) => void) => void): (arg1: T1, arg2: T2) => Promise<TResult>;
-declare function promisify<T1, T2>(fn: (arg1: T1, arg2: T2, callback: (err?: Error | null) => void) => void): (arg1: T1, arg2: T2) => Promise<void>;
-declare function promisify<T1, T2, T3, TResult>(fn: (arg1: T1, arg2: T2, arg3: T3, callback: (err: Error | null, result: TResult) => void) => void): (arg1: T1, arg2: T2, arg3: T3) => Promise<TResult>;
-declare function promisify<T1, T2, T3>(fn: (arg1: T1, arg2: T2, arg3: T3, callback: (err?: Error | null) => void) => void): (arg1: T1, arg2: T2, arg3: T3) => Promise<void>;
 declare function promisify<T1, T2, T3, T4, TResult>(
-    fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, callback: (err: Error | null, result: TResult) => void) => void,
+	fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, callback: (err: any, result: TResult) => void) => void,
 ): (arg1: T1, arg2: T2, arg3: T3, arg4: T4) => Promise<TResult>;
-declare function promisify<T1, T2, T3, T4>(fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, callback: (err?: Error | null) => void) => void): (arg1: T1, arg2: T2, arg3: T3, arg4: T4) => Promise<void>;
+declare function promisify<T1, T2, T3, T4>(
+	fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, callback: (err?: any) => void) => void,
+): (arg1: T1, arg2: T2, arg3: T3, arg4: T4) => Promise<void>;
+
 declare function promisify<T1, T2, T3, T4, T5, TResult>(
-    fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, callback: (err: Error | null, result: TResult) => void) => void,
+	fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, callback: (err: any, result: TResult) => void) => void,
 ): (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5) => Promise<TResult>;
 declare function promisify<T1, T2, T3, T4, T5>(
-    fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, callback: (err?: Error | null) => void) => void,
+	fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, callback: (err?: any) => void) => void,
 ): (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5) => Promise<void>;
+
+// tslint:disable-next-line: ban-types
 declare function promisify(fn: Function): Function;
 
 declare namespace promisify {
-    function getPolyfill(): typeof promisify;
-    const implementation: typeof promisify;
-    function shim(): typeof promisify;
+	const custom: typeof polyfill.custom;
+
+	/**
+	 * @deprecated
+	 * Not exposed by native `util.promisify` or supported by browserify's `util.promisify`.
+	 *
+	 * Use `util.promisify.custom` instead.
+	 */
+	const customPromisifyArgs: typeof polyfill.customPromisifyArgs | undefined;
+
+	function getPolyfill(): ReturnType<typeof getUtilPromisify>;
+	const implementation: typeof polyfill;
+	function shim(): ReturnType<typeof shimUtilPromisify>;
 }
 
 export = promisify;
diff --git a/types/util.promisify/polyfill.d.ts b/types/util.promisify/polyfill.d.ts
index 5056d4e9f4..bef277c3a7 100644
--- a/types/util.promisify/polyfill.d.ts
+++ b/types/util.promisify/polyfill.d.ts
@@ -1,29 +1,5 @@
-/// <reference types="node" />
-
-// tslint:disable:ban-types max-line-length
-interface CustomPromisify<TCustom extends Function> extends Function {
-    __promisify__: TCustom;
-}
-
-declare function getPolyfill<TCustom extends Function>(): (fn: CustomPromisify<TCustom>) => TCustom;
-declare function getPolyfill<TResult>(): (fn: (callback: (err: Error | null, result: TResult) => void) => void) => () => Promise<TResult>;
-declare function getPolyfill(): (fn: (callback: (err?: Error | null) => void) => void) => () => Promise<void>;
-declare function getPolyfill<T1, TResult>(): (fn: (arg1: T1, callback: (err: Error | null, result: TResult) => void) => void) => (arg1: T1) => Promise<TResult>;
-declare function getPolyfill<T1>(): (fn: (arg1: T1, callback: (err?: Error | null) => void) => void) => (arg1: T1) => Promise<void>;
-declare function getPolyfill<T1, T2, TResult>(): (fn: (arg1: T1, arg2: T2, callback: (err: Error | null, result: TResult) => void) => void) => (arg1: T1, arg2: T2) => Promise<TResult>;
-declare function getPolyfill<T1, T2>(): (fn: (arg1: T1, arg2: T2, callback: (err?: Error | null) => void) => void) => (arg1: T1, arg2: T2) => Promise<void>;
-declare function getPolyfill<T1, T2, T3, TResult>(): (fn: (arg1: T1, arg2: T2, arg3: T3, callback: (err: Error | null, result: TResult) => void) => void) => (arg1: T1, arg2: T2, arg3: T3) => Promise<TResult>;
-declare function getPolyfill<T1, T2, T3>(): (fn: (arg1: T1, arg2: T2, arg3: T3, callback: (err?: Error | null) => void) => void) => (arg1: T1, arg2: T2, arg3: T3) => Promise<void>;
-declare function getPolyfill<T1, T2, T3, T4, TResult>(): (
-    fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, callback: (err: Error | null, result: TResult) => void) => void,
-) => (arg1: T1, arg2: T2, arg3: T3, arg4: T4) => Promise<TResult>;
-declare function getPolyfill<T1, T2, T3, T4>(): (fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, callback: (err?: Error | null) => void) => void) => (arg1: T1, arg2: T2, arg3: T3, arg4: T4) => Promise<void>;
-declare function getPolyfill<T1, T2, T3, T4, T5, TResult>(): (
-    fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, callback: (err: Error | null, result: TResult) => void) => void,
-) => (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5) => Promise<TResult>;
-declare function getPolyfill<T1, T2, T3, T4, T5>(): (
-    fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, callback: (err?: Error | null) => void) => void,
-) => (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5) => Promise<void>;
-declare function getPolyfill(): (fn: Function) => Function;
+import promisify = require('./implementation');
+import util = require('util');
 
+declare function getPolyfill(): typeof promisify | typeof util.promisify;
 export = getPolyfill;
diff --git a/types/util.promisify/shim.d.ts b/types/util.promisify/shim.d.ts
index 316dc5e996..d478c12bb6 100644
--- a/types/util.promisify/shim.d.ts
+++ b/types/util.promisify/shim.d.ts
@@ -1,3 +1,4 @@
-import getPolyfill = require("./polyfill");
+import getPolyfill = require('./polyfill');
 
-export = getPolyfill;
+declare function shimUtilPromisify(): ReturnType<typeof getPolyfill>;
+export = shimUtilPromisify;
diff --git a/types/util.promisify/tsconfig.json b/types/util.promisify/tsconfig.json
index 041c10a94c..bab70d0aab 100644
--- a/types/util.promisify/tsconfig.json
+++ b/types/util.promisify/tsconfig.json
@@ -1,23 +1,19 @@
 {
-    "compilerOptions": {
-        "module": "commonjs",
-        "lib": [
-            "es6"
-        ],
-        "noImplicitAny": true,
-        "noImplicitThis": true,
-        "strictNullChecks": true,
-        "strictFunctionTypes": true,
-        "baseUrl": "../",
-        "typeRoots": [
-            "../"
-        ],
-        "types": [],
-        "noEmit": true,
-        "forceConsistentCasingInFileNames": true
-    },
-    "files": [
-        "index.d.ts",
-        "util.promisify-tests.ts"
-    ]
-}
\ No newline at end of file
+	"compilerOptions": {
+		"module": "commonjs",
+		"lib": ["es2015"],
+		"noImplicitAny": true,
+		"noImplicitThis": true,
+		"strictNullChecks": true,
+		"strictFunctionTypes": true,
+		"baseUrl": "../",
+		"typeRoots": ["../"],
+		"types": [],
+		"noEmit": true,
+		"forceConsistentCasingInFileNames": true
+	},
+	"files": [
+		"util.promisify-tests.ts",
+		"index.d.ts"
+	]
+}
diff --git a/types/util.promisify/util.promisify-tests.ts b/types/util.promisify/util.promisify-tests.ts
index 50573765bb..93614527a7 100644
--- a/types/util.promisify/util.promisify-tests.ts
+++ b/types/util.promisify/util.promisify-tests.ts
@@ -4,12 +4,38 @@ import promisify = require('util.promisify');
 
 import 'util.promisify/auto';
 
-let readFile = promisify(fs.readFile);
-readFile = util.promisify(fs.readFile);
+/**
+ * The `expectType` function from https://www.npmjs.com/package/tsd,
+ * except instead of returning `void`, it returns `T`.
+ */
+declare function expectType<T>(value: T): T;
 
+expectType<typeof fs.readFile.__promisify__>(promisify(fs.readFile));
+expectType<typeof fs.readFile.__promisify__>(util.promisify(fs.readFile));
+
+// $ExpectType typeof promisify | typeof promisify
 let implementation = promisify.getPolyfill();
 implementation = promisify.shim();
-implementation = promisify.implementation;
 
-import promisifyShim = require('util.promisify/shim');
-promisifyShim();
+// $ExpectType typeof promisify
+const polyfillImpl = promisify.implementation;
+implementation = polyfillImpl;
+
+import getPolyfill = require('util.promisify/polyfill');
+implementation = getPolyfill();
+
+import shimUtilPromisify = require('util.promisify/shim');
+implementation = shimUtilPromisify();
+
+// The `promisify.custom` symbols are considered equal by the type system:
+
+expectType<typeof util.promisify.custom>(implementation.custom); // $ExpectType typeof custom
+expectType<typeof util.promisify.custom>(polyfillImpl.custom); // $ExpectType typeof custom
+expectType<typeof implementation.custom>(util.promisify.custom); // $ExpectType typeof custom
+expectType<typeof implementation.custom>(polyfillImpl.custom); // $ExpectType typeof custom
+expectType<typeof polyfillImpl.custom>(implementation.custom); // $ExpectType typeof custom
+expectType<typeof polyfillImpl.custom>(util.promisify.custom); // $ExpectType typeof custom
+
+promisify.customPromisifyArgs; // $ExpectType typeof customPromisifyArgs | undefined
+polyfillImpl.customPromisifyArgs; // $ExpectType typeof customPromisifyArgs
+implementation.customPromisifyArgs; // $ExpectType typeof customPromisifyArgs | undefined