diff --git a/core/lib/Drupal/Core/Test/TestSetupTrait.php b/core/lib/Drupal/Core/Test/TestSetupTrait.php
index 44409078dbf3854670711937948c404d887beb36..0625691022cd451bde0804482f593089a792f654 100644
--- a/core/lib/Drupal/Core/Test/TestSetupTrait.php
+++ b/core/lib/Drupal/Core/Test/TestSetupTrait.php
@@ -158,6 +158,9 @@ protected function changeDatabasePrefix() {
     // If the test is run with argument dburl then use it.
     $db_url = getenv('SIMPLETEST_DB');
     if (!empty($db_url)) {
+      // Ensure no existing database gets in the way. If a default database
+      // exists already it must be removed.
+      Database::removeConnection('default');
       $database = Database::convertDbUrlToConnectionInfo($db_url, isset($this->root) ? $this->root : DRUPAL_ROOT);
       Database::addConnectionInfo('default', 'default', $database);
     }
diff --git a/core/tests/Drupal/FunctionalTests/Update/UpdatePathTestBase.php b/core/tests/Drupal/FunctionalTests/Update/UpdatePathTestBase.php
index 34b4cc4dbef94ce1c6131e56658745cd48155d12..7abe0efe30a6656c0c42c2bc50c0b8bd0075d9bc 100644
--- a/core/tests/Drupal/FunctionalTests/Update/UpdatePathTestBase.php
+++ b/core/tests/Drupal/FunctionalTests/Update/UpdatePathTestBase.php
@@ -161,7 +161,15 @@ protected function setUp() {
     $kernel = TestRunnerKernel::createFromRequest($request, $autoloader);
     $kernel->loadLegacyIncludes();
 
-    $this->changeDatabasePrefix();
+    // Set the update url. This must be set here rather than in
+    // self::__construct() or the old URL generator will leak additional test
+    // sites.
+    $this->updateUrl = Url::fromRoute('system.db_update');
+
+    $this->setupBaseUrl();
+
+    // Install Drupal test site.
+    $this->prepareEnvironment();
     $this->runDbTasks();
     // Allow classes to set database dump files.
     $this->setDatabaseDumpFiles();
@@ -173,15 +181,6 @@ protected function setUp() {
       parent::setUp();
       return;
     }
-    // Set the update url. This must be set here rather than in
-    // self::__construct() or the old URL generator will leak additional test
-    // sites.
-    $this->updateUrl = Url::fromRoute('system.db_update');
-
-    $this->setupBaseUrl();
-
-    // Install Drupal test site.
-    $this->prepareEnvironment();
     $this->installDrupal();
 
     // Add the config directories to settings.php.
diff --git a/core/tests/Drupal/Tests/Core/Test/TestSetupTraitTest.php b/core/tests/Drupal/Tests/Core/Test/TestSetupTraitTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..d0b82835666100dec9f39f2944eb4d1c2a59a566
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Test/TestSetupTraitTest.php
@@ -0,0 +1,50 @@
+<?php
+
+namespace Drupal\Tests\Core\Test;
+
+use Drupal\Core\Database\Database;
+use Drupal\Core\Test\TestSetupTrait;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests the TestSetupTrait trait.
+ *
+ * @coversDefaultClass \Drupal\Core\Test\TestSetupTrait
+ * @group Testing
+ *
+ * Run in a separate process as this test involves Database statics and
+ * environment variables.
+ * @runTestsInSeparateProcesses
+ * @preserveGlobalState disabled
+ */
+class TestSetupTraitTest extends UnitTestCase {
+
+  /**
+   * Tests the SIMPLETEST_DB environment variable is used.
+   *
+   * @covers ::changeDatabasePrefix
+   */
+  public function testChangeDatabasePrefix() {
+    putenv('SIMPLETEST_DB=pgsql://user:pass@127.0.0.1/db');
+    $connection_info = Database::convertDbUrlToConnectionInfo('mysql://user:pass@localhost/db', '');
+    Database::addConnectionInfo('default', 'default', $connection_info);
+    $this->assertEquals('mysql', Database::getConnectionInfo()['default']['driver']);
+    $this->assertEquals('localhost', Database::getConnectionInfo()['default']['host']);
+
+    // Create a mock for testing the trait and set a few properties that are
+    // used to avoid unnecessary set up.
+    $test_setup = $this->getMockForTrait(TestSetupTrait::class);
+    $test_setup->databasePrefix = 'testDbPrefix';
+    $test_setup->root = '';
+
+    $method = new \ReflectionMethod(get_class($test_setup), 'changeDatabasePrefix');
+    $method->setAccessible(TRUE);
+    $method->invoke($test_setup);
+
+    // Ensure that SIMPLETEST_DB defines the default database connection after
+    // calling \Drupal\Core\Test\TestSetupTrait::changeDatabasePrefix().
+    $this->assertEquals('pgsql', Database::getConnectionInfo()['default']['driver']);
+    $this->assertEquals('127.0.0.1', Database::getConnectionInfo()['default']['host']);
+  }
+
+}