diff -u b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -759,6 +759,12 @@
    * and reset the database prefix.
    */
   protected function tearDown() {
+    // Ensure that TestBase::changeDatabasePrefix() has run and TestBase::$setup
+    // was not tricked into TRUE, since the following code would delete the
+    // entire parent site otherwise.
+    if (!$this->setupDatabasePrefix) {
+      return FALSE;
+    }
     // Remove all prefixed tables.
     $connection_info = Database::getConnectionInfo('default');
     $tables = db_find_tables($connection_info['default']['prefix']['default'] . '%');