diff --git a/src/wp-includes/user.php b/src/wp-includes/user.php
index 867df81e166..fce40a91bae 100644
--- a/src/wp-includes/user.php
+++ b/src/wp-includes/user.php
@@ -2123,10 +2123,16 @@ function wp_insert_user( $userdata ) {
 		return new WP_Error( 'user_login_too_long', __( 'Username may not be longer than 60 characters.' ) );
 	}
 
+	// Username must be unique.
 	if ( ! $update && username_exists( $user_login ) ) {
 		return new WP_Error( 'existing_user_login', __( 'Sorry, that username already exists!' ) );
 	}
 
+	// Username must not exist as an existing email.
+	if ( email_exists( $user_login ) ) {
+		return new WP_Error( 'existing_user_login_as_email', __( 'Sorry, that username is not available.' ) );
+	}
+
 	/**
 	 * Filters the list of disallowed usernames.
 	 *
@@ -3340,7 +3346,8 @@ function register_new_user( $user_login, $user_email ) {
 		$sanitized_user_login = '';
 	} elseif ( username_exists( $sanitized_user_login ) ) {
 		$errors->add( 'username_exists', __( '<strong>Error:</strong> This username is already registered. Please choose another one.' ) );
-
+	} elseif ( email_exists( $sanitized_user_login ) ) {
+		$errors->add( 'username_exists_as_email', __( '<strong>Error:</strong> This username is not available. Please choose another one.' ) );
 	} else {
 		/** This filter is documented in wp-includes/user.php */
 		$illegal_user_logins = (array) apply_filters( 'illegal_user_logins', array() );
diff --git a/tests/phpunit/tests/user.php b/tests/phpunit/tests/user.php
index 7157a00cfa9..5ebd91f690f 100644
--- a/tests/phpunit/tests/user.php
+++ b/tests/phpunit/tests/user.php
@@ -934,6 +934,24 @@ public function test_wp_insert_user_should_not_generate_user_nicename_longer_tha
 		$this->assertSame( $expected, $user->user_nicename );
 	}
 
+	/**
+	 * @ticket 57394
+	 */
+	public function test_wp_insert_user_should_reject_username_that_matches_existing_user_email() {
+		$existing_email = get_option( 'admin_email' );
+		$user           = wp_insert_user(
+			array(
+				'user_login'    => $existing_email,
+				'user_email'    => 'whatever@example.com',
+				'user_pass'     => 'whatever',
+				'user_nicename' => 'whatever',
+			)
+		);
+
+		$this->assertWPError( $user );
+		$this->assertSame( 'existing_user_login_as_email', $user->get_error_code() );
+	}
+
 	/**
 	 * @ticket 33793
 	 */