diff --git a/composer.json b/composer.json
index e7228a32ad..c753be2ab0 100644
--- a/composer.json
+++ b/composer.json
@@ -19,20 +19,6 @@
 		"phpcompatibility/phpcompatibility-wp": "~2.1.2",
 		"phpunit/phpunit": "^5.7 || ^6.5 || ^7.5"
 	},
-	"autoload-dev": {
-		"files": [
-			"tests/phpunit/includes/phpunit7/MockObject/Builder/NamespaceMatch.php",
-			"tests/phpunit/includes/phpunit7/MockObject/Builder/ParametersMatch.php",
-			"tests/phpunit/includes/phpunit7/MockObject/InvocationMocker.php",
-			"tests/phpunit/includes/phpunit7/MockObject/MockMethod.php"
-		],
-		"exclude-from-classmap": [
-			"vendor/phpunit/phpunit/src/Framework/MockObject/Builder/NamespaceMatch.php",
-			"vendor/phpunit/phpunit/src/Framework/MockObject/Builder/ParametersMatch.php",
-			"vendor/phpunit/phpunit/src/Framework/MockObject/InvocationMocker.php",
-			"vendor/phpunit/phpunit/src/Framework/MockObject/MockMethod.php"
-		]
-	},
 	"scripts": {
 		"compat": "@php ./vendor/squizlabs/php_codesniffer/bin/phpcs --standard=phpcompat.xml.dist --report=summary,source",
 		"format": "@php ./vendor/squizlabs/php_codesniffer/bin/phpcbf --report=summary,source",
diff --git a/tests/phpunit/includes/bootstrap.php b/tests/phpunit/includes/bootstrap.php
index 6419cd2890..160a7f7a4d 100644
--- a/tests/phpunit/includes/bootstrap.php
+++ b/tests/phpunit/includes/bootstrap.php
@@ -46,6 +46,10 @@ if ( version_compare( $phpunit_version, '5.7', '<' ) || version_compare( $phpuni
 	exit( 1 );
 }
 
+// Register a custom autoloader for the PHPUnit 9.x Mockobject classes for compatibility with PHP 8.0.
+require_once __DIR__ . '/class-mockobject-autoload.php';
+spl_autoload_register( 'MockObject_Autoload::load', true, true );
+
 // If running core tests, check if all the required PHP extensions are loaded before running the test suite.
 if ( defined( 'WP_RUN_CORE_TESTS' ) && WP_RUN_CORE_TESTS ) {
 	$required_extensions = array(
diff --git a/tests/phpunit/includes/class-mockobject-autoload.php b/tests/phpunit/includes/class-mockobject-autoload.php
new file mode 100644
index 0000000000..b8f46e8a2e
--- /dev/null
+++ b/tests/phpunit/includes/class-mockobject-autoload.php
@@ -0,0 +1,63 @@
+<?php
+
+/**
+ * Custom autoloader for the PHPUnit 9.x MockObject classes.
+ *
+ * Hack around PHPUnit < 9.3 mocking not being compatible with PHP 8.
+ *
+ * This is a temporary solution until the PHPUnit version constraints are widened.
+ *
+ * @since 5.9.0
+ */
+final class MockObject_Autoload {
+
+	/**
+	 * A list of the classes this autoloader handles.
+	 *
+	 * @var string[] => true
+	 */
+	private static $supported_classes = array(
+		'PHPUnit\Framework\MockObject\Builder\NamespaceMatch' => '/phpunit7/MockObject/Builder/NamespaceMatch.php',
+		'PHPUnit\Framework\MockObject\Builder\ParametersMatch' => '/phpunit7/MockObject/Builder/ParametersMatch.php',
+		'PHPUnit\Framework\MockObject\InvocationMocker' => '/phpunit7/MockObject/InvocationMocker.php',
+		'PHPUnit\Framework\MockObject\MockMethod'       => '/phpunit7/MockObject/MockMethod.php',
+	);
+
+	/**
+	 * Loads a class.
+	 *
+	 * @param string $class_name The name of the class to load.
+	 * @return bool
+	 */
+	public static function load( $class_name ) {
+
+		if ( isset( self::$supported_classes[ $class_name ] ) === false ) {
+			// Bow out, not a class this autoloader handles.
+			return false;
+		}
+
+		if ( PHP_VERSION_ID < 80000 ) {
+			// This autoloader is only needed when the tests are being run on PHP >= 8.0.
+			// Let the standard Composer autoloader handle things otherwise.
+			return false;
+		}
+
+		if ( class_exists( 'PHPUnit\Runner\Version' ) === false // PHPUnit < 6.0.
+			|| ( version_compare( \PHPUnit\Runner\Version::id(), '7.0.0', '<' )
+			&& version_compare( \PHPUnit\Runner\Version::id(), '8.0.0', '>=' ) )
+		) {
+			// This autoloader is only needed when the tests are being run on PHPUnit 7.
+			return false;
+		}
+
+		$relative_path = self::$supported_classes[ $class_name ];
+		$file          = \realpath( __DIR__ . $relative_path );
+
+		if ( false === $file || @\file_exists( $file ) === false ) {
+			return false;
+		}
+
+		require_once $file;
+		return true;
+	}
+}