diff --git a/tests/phpunit/includes/install.php b/tests/phpunit/includes/install.php
index 8ce307818d..723206d7df 100644
--- a/tests/phpunit/includes/install.php
+++ b/tests/phpunit/includes/install.php
@@ -14,6 +14,13 @@ if ( ! defined( 'WP_RUN_CORE_TESTS' ) && in_array( 'run_core_tests', $argv, true
 }
 
 define( 'WP_INSTALLING', true );
+
+/*
+ * Cron tries to make an HTTP request to the site, which always fails,
+ * because tests are run in CLI mode only.
+ */
+define( 'DISABLE_WP_CRON', true );
+
 require_once $config_file_path;
 require_once __DIR__ . '/functions.php';