diff --git a/expect/_equal.ts b/expect/_equal.ts
index af6c7fa01392..ec022203486c 100644
--- a/expect/_equal.ts
+++ b/expect/_equal.ts
@@ -19,15 +19,7 @@ function constructorsEqual(a: object, b: object) {
  * Deep equality comparison used in assertions
  * @param c actual value
  * @param d expected value
- * @param strictCheck check value in strictMode
- *
- * @example
- * ```ts
- * import { equal } from "https://deno.land/std@$STD_VERSION/assert/equal.ts";
- *
- * equal({ foo: "bar" }, { foo: "bar" }); // Returns `true`
- * equal({ foo: "bar" }, { foo: "baz" }); // Returns `false
- * ```
+ * @param options for the equality check
  */
 export function equal(c: unknown, d: unknown, options?: EqualOptions): boolean {
   const { customTesters = [], strictCheck } = options || {};
@@ -36,7 +28,10 @@ export function equal(c: unknown, d: unknown, options?: EqualOptions): boolean {
   return (function compare(a: unknown, b: unknown): boolean {
     if (customTesters?.length) {
       for (const customTester of customTesters) {
-        const pass = customTester.call(undefined, a, b, customTesters);
+        const testContext = {
+          equal,
+        };
+        const pass = customTester.call(testContext, a, b, customTesters);
         if (pass !== undefined) {
           return pass;
         }
diff --git a/expect/_extend.ts b/expect/_extend.ts
new file mode 100644
index 000000000000..b5bc42d14792
--- /dev/null
+++ b/expect/_extend.ts
@@ -0,0 +1,16 @@
+// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
+
+import { Matchers } from "./_types.ts";
+
+let extendMatchers = {};
+
+export function getExtendMatchers() {
+  return extendMatchers;
+}
+
+export function setExtendMatchers(newExtendMatchers: Matchers) {
+  extendMatchers = {
+    ...extendMatchers,
+    ...newExtendMatchers,
+  };
+}
diff --git a/expect/_extend_test.ts b/expect/_extend_test.ts
new file mode 100644
index 000000000000..8689773c79d2
--- /dev/null
+++ b/expect/_extend_test.ts
@@ -0,0 +1,100 @@
+// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
+
+import { expect } from "./expect.ts";
+import { MatcherContext, Tester } from "./_types.ts";
+
+declare module "./_types.ts" {
+  interface Expected {
+    toEqualBook: (expected: unknown) => ExtendMatchResult;
+  }
+}
+
+class Author {
+  public name: string;
+
+  constructor(name: string) {
+    this.name = name;
+  }
+}
+
+class Book {
+  public name: string;
+  public authors: Array<Author>;
+
+  constructor(name: string, authors: Array<Author>) {
+    this.name = name;
+    this.authors = authors;
+  }
+}
+
+const areAuthorsEqual: Tester = (a: unknown, b: unknown) => {
+  const isAAuthor = a instanceof Author;
+  const isBAuthor = b instanceof Author;
+
+  if (isAAuthor && isBAuthor) {
+    return a.name === b.name;
+  } else if (isAAuthor === isBAuthor) {
+    return undefined;
+  } else {
+    return false;
+  }
+};
+
+const areBooksEqual: Tester = function (
+  this: MatcherContext,
+  a: unknown,
+  b: unknown,
+  customTesters: Tester[],
+) {
+  const isABook = a instanceof Book;
+  const isBBook = b instanceof Book;
+
+  if (isABook && isBBook) {
+    return (a.name === b.name &&
+      this.equal(a.authors, b.authors, { customTesters: customTesters }));
+  } else if (isABook === isBBook) {
+    return undefined;
+  } else {
+    return false;
+  }
+};
+
+expect.addEqualityTesters([
+  areAuthorsEqual,
+  areBooksEqual,
+]);
+
+expect.extend({
+  toEqualBook(context, expected) {
+    const actual = context.value as Book;
+    const result = context.equal(expected, actual, {
+      customTesters: context.customTesters,
+    });
+
+    return {
+      message: () =>
+        `Expected Book object: ${expected.name}. Actual Book object: ${actual.name}`,
+      pass: result,
+    };
+  },
+});
+
+Deno.test("expect.extend() api test case", () => {
+  const book1a = new Book("Book 1", [
+    new Author("Author 1"),
+    new Author("Author 2"),
+  ]);
+  const book1b = new Book("Book 1", [
+    new Author("Author 1"),
+    new Author("Author 2"),
+  ]);
+  const book2 = new Book("Book 2", [
+    new Author("Author 1"),
+    new Author("Author 2"),
+  ]);
+
+  expect(book1a).toEqualBook(book1b);
+  expect(book1a).not.toEqualBook(book2);
+  expect(book1a).not.toEqualBook(1);
+  expect(book1a).not.toEqualBook(null);
+});
diff --git a/expect/_types.ts b/expect/_types.ts
index 9a001d06d0a8..6af7997ac40f 100644
--- a/expect/_types.ts
+++ b/expect/_types.ts
@@ -4,6 +4,7 @@
 export interface MatcherContext {
   value: unknown;
   isNot: boolean;
+  equal: (a: unknown, b: unknown, options?: EqualOptions) => boolean;
   customTesters: Tester[];
   customMessage: string | undefined;
 }
@@ -11,12 +12,16 @@ export interface MatcherContext {
 export type Matcher = (
   context: MatcherContext,
   ...args: any[]
-) => MatchResult;
+) => MatchResult | ExtendMatchResult;
 
 export type Matchers = {
   [key: string]: Matcher;
 };
 export type MatchResult = void | Promise<void> | boolean;
+export type ExtendMatchResult = {
+  message: () => string;
+  pass: boolean;
+};
 export type AnyConstructor = new (...args: any[]) => any;
 
 export type Tester = (
@@ -88,6 +93,9 @@ export interface Expected {
   not: Expected;
   resolves: Async<Expected>;
   rejects: Async<Expected>;
+  // This declaration prepares for the `expect.extend` and just only let
+  // compiler pass, the more concrete type definition is defined by user
+  [name: string]: unknown;
 }
 
 export type MatcherKey = keyof Omit<Expected, "not" | "resolves" | "rejects">;
diff --git a/expect/expect.ts b/expect/expect.ts
index c5bc4af67725..89090612fbbb 100644
--- a/expect/expect.ts
+++ b/expect/expect.ts
@@ -3,15 +3,19 @@
 
 import type {
   Expected,
+  ExtendMatchResult,
   Matcher,
   MatcherContext,
   MatcherKey,
+  Matchers,
 } from "./_types.ts";
 import { AssertionError } from "../assert/assertion_error.ts";
 import {
   addCustomEqualityTesters,
   getCustomEqualityTesters,
 } from "./_custom_equality_tester.ts";
+import { equal } from "./_equal.ts";
+import { getExtendMatchers, setExtendMatchers } from "./_extend.ts";
 import {
   toBe,
   toBeCloseTo,
@@ -132,7 +136,12 @@ export function expect(value: unknown, customMessage?: string): Expected {
           return self;
         }
 
-        const matcher: Matcher = matchers[name as MatcherKey];
+        const extendMatchers: Matchers = getExtendMatchers();
+        const allMatchers = {
+          ...extendMatchers,
+          ...matchers,
+        };
+        const matcher: Matcher = allMatchers[name as MatcherKey];
         if (!matcher) {
           throw new TypeError(
             typeof name === "string"
@@ -145,6 +154,7 @@ export function expect(value: unknown, customMessage?: string): Expected {
           function applyMatcher(value: unknown, args: unknown[]) {
             const context: MatcherContext = {
               value,
+              equal,
               isNot: false,
               customMessage,
               customTesters: getCustomEqualityTesters(),
@@ -152,7 +162,18 @@ export function expect(value: unknown, customMessage?: string): Expected {
             if (isNot) {
               context.isNot = true;
             }
-            matcher(context, ...args);
+            if (name in extendMatchers) {
+              const result = matcher(context, ...args) as ExtendMatchResult;
+              if (context.isNot) {
+                if (result.pass) {
+                  throw new AssertionError(result.message());
+                }
+              } else if (!result.pass) {
+                throw new AssertionError(result.message());
+              }
+            } else {
+              matcher(context, ...args);
+            }
           }
 
           return isPromised
@@ -169,6 +190,7 @@ export function expect(value: unknown, customMessage?: string): Expected {
 }
 
 expect.addEqualityTesters = addCustomEqualityTesters;
+expect.extend = setExtendMatchers;
 expect.anything = anything;
 expect.any = any;
 expect.arrayContaining = arrayContaining;