diff --git a/tests/phpunit/includes/bootstrap.php b/tests/phpunit/includes/bootstrap.php
index 81910e0701..65c1c370c4 100644
--- a/tests/phpunit/includes/bootstrap.php
+++ b/tests/phpunit/includes/bootstrap.php
@@ -77,7 +77,10 @@ define( 'DIR_TESTDATA', __DIR__ . '/../data' );
 define( 'DIR_TESTROOT', realpath( dirname( __DIR__ ) ) );
 
 define( 'WP_LANG_DIR', realpath( DIR_TESTDATA . '/languages' ) );
-define( 'WP_PLUGIN_DIR', realpath( DIR_TESTDATA . '/plugins' ) );
+
+if ( defined( 'WP_RUN_CORE_TESTS' ) && WP_RUN_CORE_TESTS ) {
+	define( 'WP_PLUGIN_DIR', realpath( DIR_TESTDATA . '/plugins' ) );
+}
 
 if ( ! defined( 'WP_TESTS_FORCE_KNOWN_BUGS' ) ) {
 	define( 'WP_TESTS_FORCE_KNOWN_BUGS', false );