diff --git a/core/lib/Drupal/Core/Database/Database.php b/core/lib/Drupal/Core/Database/Database.php
index 6439c17dba..d4b5aab86e 100644
--- a/core/lib/Drupal/Core/Database/Database.php
+++ b/core/lib/Drupal/Core/Database/Database.php
@@ -261,6 +261,44 @@ final public static function parseConnectionInfo(array $info) {
       $info['namespace'] = 'Drupal\\' . $info['driver'] . '\\Driver\\Database\\' . $info['driver'];
     }
 
+    // Backwards compatibility layer for Drupal 8 style database connection
+    // arrays. Those have the wrong 'namespace' key set, or not set at all
+    // for core supported database drivers.
+    if (strpos($info['namespace'], 'Drupal\\Core\\Database\\Driver\\') === 0) {
+      switch (strtolower($info['driver'])) {
+        case 'mysql':
+          $info['namespace'] = 'Drupal\\mysql\\Driver\\Database\\mysql';
+          break;
+
+        case 'pgsql':
+          $info['namespace'] = 'Drupal\\pgsql\\Driver\\Database\\pgsql';
+          break;
+
+        case 'sqlite':
+          $info['namespace'] = 'Drupal\\sqlite\\Driver\\Database\\sqlite';
+          break;
+      }
+    }
+
+    // Backwards compatibility layer for Drupal 8 style database connection
+    // arrays. Those do not have the 'autoload' key set for core database
+    // drivers.
+    if (empty($info['autoload'])) {
+      switch (trim($info['namespace'], '\\')) {
+        case "Drupal\\mysql\\Driver\\Database\\mysql":
+          $info['autoload'] = "core/modules/mysql/src/Driver/Database/mysql/";
+          break;
+
+        case "Drupal\\pgsql\\Driver\\Database\\pgsql":
+          $info['autoload'] = "core/modules/pgsql/src/Driver/Database/pgsql/";
+          break;
+
+        case "Drupal\\sqlite\\Driver\\Database\\sqlite":
+          $info['autoload'] = "core/modules/sqlite/src/Driver/Database/sqlite/";
+          break;
+      }
+    }
+
     return $info;
   }
 
@@ -286,12 +324,26 @@ final public static function parseConnectionInfo(array $info) {
    *   The database connection information, as defined in settings.php. The
    *   structure of this array depends on the database driver it is connecting
    *   to.
+   * @param \Composer\Autoload\ClassLoader $class_loader
+   *   The class loader. Used for adding the database driver to the autoloader
+   *   if $info['autoload'] is set.
    *
    * @see \Drupal\Core\Database\Database::setActiveConnection
    */
-  final public static function addConnectionInfo($key, $target, array $info) {
+  final public static function addConnectionInfo($key, $target, array $info, $class_loader = NULL) {
     if (empty(self::$databaseInfo[$key][$target])) {
-      self::$databaseInfo[$key][$target] = self::parseConnectionInfo($info);
+      $info = self::parseConnectionInfo($info);
+      self::$databaseInfo[$key][$target] = $info;
+
+      // The database driver needs to be loadable before module namespaces have
+      // been added to the autoloader, because the list of enabled modules and
+      // the service container definition are typically both in the database.
+      if (isset($info['autoload'])) {
+        if (!$class_loader) {
+          $class_loader = self::getClassLoader();
+        }
+        $class_loader->addPsr4($info['namespace'] . '\\', $info['autoload']);
+      }
     }
   }
 
@@ -324,11 +376,17 @@ final public static function getAllConnectionInfo() {
    * @param array $databases
    *   A multi-dimensional array specifying database connection parameters, as
    *   defined in settings.php.
+   * @param \Composer\Autoload\ClassLoader $class_loader
+   *   The class loader. Used for adding the database driver(s) to the
+   *   autoloader if $databases[$key][$target]['autoload'] is set.
    */
-  final public static function setMultipleConnectionInfo(array $databases) {
+  final public static function setMultipleConnectionInfo(array $databases, $class_loader = NULL) {
+    if (!$class_loader) {
+      $class_loader = self::getClassLoader();
+    }
     foreach ($databases as $key => $targets) {
       foreach ($targets as $target => $info) {
-        self::addConnectionInfo($key, $target, $info);
+        self::addConnectionInfo($key, $target, $info, $class_loader);
       }
     }
   }
@@ -701,4 +759,23 @@ private static function isWithinModuleNamespace(string $namespace) {
     return ($first === 'Drupal' && strtolower($second) === $second);
   }
 
+  /**
+   * Returns either Drupal's primary class loader or a separate one.
+   *
+   * @return \Composer\Autoload\ClassLoader
+   *   A class loader into which a database driver's namespace can be added if
+   *   it requires autoloading.
+   */
+  private static function getClassLoader() {
+    // Typically, the container isn't initialized until after the default
+    // database connection has been opened, but use the service if it's there.
+    if (\Drupal::hasContainer() && \Drupal::hasService('class_loader')) {
+      return \Drupal::service('class_loader');
+    }
+
+    $additional_class_loader = new ClassLoader();
+    $additional_class_loader->register(TRUE);
+    return $additional_class_loader;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Site/Settings.php b/core/lib/Drupal/Core/Site/Settings.php
index 31d494f8b4..7563109b6b 100644
--- a/core/lib/Drupal/Core/Site/Settings.php
+++ b/core/lib/Drupal/Core/Site/Settings.php
@@ -159,58 +159,7 @@ public static function initialize($app_root, $site_path, &$class_loader) {
 
     self::handleDeprecations($settings);
 
-    // Initialize databases.
-    foreach ($databases as $key => $targets) {
-      foreach ($targets as $target => $info) {
-        // Backwards compatibility layer for Drupal 8 style database connection
-        // arrays. Those have the wrong 'namespace' key set, or not set at all
-        // for core supported database drivers.
-        if (empty($info['namespace']) || (strpos($info['namespace'], 'Drupal\\Core\\Database\\Driver\\') === 0)) {
-          switch (strtolower($info['driver'])) {
-            case 'mysql':
-              $info['namespace'] = 'Drupal\\mysql\\Driver\\Database\\mysql';
-              break;
-
-            case 'pgsql':
-              $info['namespace'] = 'Drupal\\pgsql\\Driver\\Database\\pgsql';
-              break;
-
-            case 'sqlite':
-              $info['namespace'] = 'Drupal\\sqlite\\Driver\\Database\\sqlite';
-              break;
-          }
-        }
-        // Backwards compatibility layer for Drupal 8 style database connection
-        // arrays. Those do not have the 'autoload' key set for core database
-        // drivers.
-        if (empty($info['autoload'])) {
-          switch (trim($info['namespace'], '\\')) {
-            case "Drupal\\mysql\\Driver\\Database\\mysql":
-              $info['autoload'] = "core/modules/mysql/src/Driver/Database/mysql/";
-              break;
-
-            case "Drupal\\pgsql\\Driver\\Database\\pgsql":
-              $info['autoload'] = "core/modules/pgsql/src/Driver/Database/pgsql/";
-              break;
-
-            case "Drupal\\sqlite\\Driver\\Database\\sqlite":
-              $info['autoload'] = "core/modules/sqlite/src/Driver/Database/sqlite/";
-              break;
-          }
-        }
-
-        Database::addConnectionInfo($key, $target, $info);
-        // If the database driver is provided by a module, then its code may
-        // need to be instantiated prior to when the module's root namespace
-        // is added to the autoloader, because that happens during service
-        // container initialization but the container definition is likely in
-        // the database. Therefore, allow the connection info to specify an
-        // autoload directory for the driver.
-        if (isset($info['autoload'])) {
-          $class_loader->addPsr4($info['namespace'] . '\\', $info['autoload']);
-        }
-      }
-    }
+    Database::setMultipleConnectionInfo($databases, $class_loader);
 
     // Initialize Settings.
     new Settings($settings);
diff --git a/core/tests/Drupal/Tests/Core/Database/UrlConversionTest.php b/core/tests/Drupal/Tests/Core/Database/UrlConversionTest.php
index fb1c9dde8d..5b41dbcbb6 100644
--- a/core/tests/Drupal/Tests/Core/Database/UrlConversionTest.php
+++ b/core/tests/Drupal/Tests/Core/Database/UrlConversionTest.php
@@ -32,8 +32,10 @@ protected function setUp(): void {
     $container = $this->createMock('Symfony\Component\DependencyInjection\ContainerInterface');
     $container->expects($this->any())
       ->method('has')
-      ->with('kernel')
-      ->willReturn(TRUE);
+      ->will($this->returnValueMap([
+        ['kernel', TRUE],
+        ['class_loader', FALSE],
+      ]));
     $container->expects($this->any())
       ->method('getParameter')
       ->with('site.path')
diff --git a/core/tests/Drupal/Tests/Core/Site/SettingsTest.php b/core/tests/Drupal/Tests/Core/Site/SettingsTest.php
index bcc2eda6c9..662c33a51a 100644
--- a/core/tests/Drupal/Tests/Core/Site/SettingsTest.php
+++ b/core/tests/Drupal/Tests/Core/Site/SettingsTest.php
@@ -403,4 +403,46 @@ public function providerTestDatabaseInfoInitialization(): array {
     ];
   }
 
+  /**
+   * Tests that code within settings.php can load database driver classes.
+   */
+  public function testDatabaseAccessWithinSettingsPhp(): void {
+    $class_exists = NULL;
+
+    // During PHPUnit tests, the current working directory might not be
+    // DRUPAL_ROOT, but during normal Drupal execution it is, and some calls
+    // to \Composer\Autoload\ClassLoader::addPsr4() assume it is by passing in
+    // paths that are relative to it.
+    $original_cwd = getcwd();
+    chdir(DRUPAL_ROOT);
+
+    // Ensure the driver isn't loadable yet.
+    $this->assertFalse(class_exists('Drupal\\Driver\\Database\\fake\\Connection'));
+
+    // Create a mock settings.php file and require it.
+    $databases['mock']['default'] = [
+      'driver' => 'fake',
+      'namespace' => 'Drupal\\Driver\\Database\\fake',
+      'autoload' => 'core/tests/fixtures/database_drivers/custom/fake/',
+    ];
+    $settings_file_content = "<?php\n\$databases = " . var_export($databases, TRUE) . ";\n";
+    $settings_file_content .= <<<'PHP'
+      \Drupal\Core\Database\Database::setMultipleConnectionInfo($databases);
+      $class_exists = class_exists('Drupal\\Driver\\Database\\fake\\Connection');
+PHP;
+
+    $vfs_root = vfsStream::setup('root');
+    $sites_directory = vfsStream::newDirectory('sites')->at($vfs_root);
+    vfsStream::newFile('settings.php')
+      ->at($sites_directory)
+      ->setContent($settings_file_content);
+    require vfsStream::url('root') . '/sites/settings.php';
+
+    // Restore PHPUnit's original working directory.
+    chdir($original_cwd);
+
+    // Ensure the database driver was loadable from within settings.php.
+    $this->assertTrue($class_exists);
+  }
+
 }