diff --git a/core/lib/Drupal/Core/Database/Database.php b/core/lib/Drupal/Core/Database/Database.php
index 6493c87c5c..3d044127fe 100644
--- a/core/lib/Drupal/Core/Database/Database.php
+++ b/core/lib/Drupal/Core/Database/Database.php
@@ -238,9 +238,41 @@ abstract class Database {
     // Prefix information, default to an empty prefix.
     $info['prefix'] = $info['prefix'] ?? '';
 
-    // Fallback for Drupal 7 settings.php if namespace is not provided.
-    if (empty($info['namespace'])) {
-      $info['namespace'] = 'Drupal\\' . $info['driver'] . '\\Driver\\Database\\' . $info['driver'];
+    // Backwards compatibility layer for Drupal 8 style database connection
+    // arrays. Those have the wrong 'namespace' key set, or not set at all
+    // for core supported database drivers.
+    if (empty($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;
+      }
     }
 
     return $info;
@@ -268,12 +300,28 @@ abstract class Database {
    *   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.
+   * @param string $app_root
+   *   The app root.
    *
    * @see \Drupal\Core\Database\Database::setActiveConnection
    */
-  final public static function addConnectionInfo($key, $target, array $info) {
+  final public static function addConnectionInfo($key, $target, array $info, $class_loader = NULL, $app_root = NULL) {
     if (empty(self::$databaseInfo[$key][$target])) {
-      self::$databaseInfo[$key][$target] = self::parseConnectionInfo($info);
+      $info = self::parseConnectionInfo($info);
+      self::$databaseInfo[$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 && $app_root) {
+        $class_loader->addPsr4($info['namespace'] . '\\', $app_root . '/' . $info['autoload']);
+      }
     }
   }
 
@@ -306,11 +354,16 @@ abstract class Database {
    * @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.
+   * @param string $app_root
+   *   The app root.
    */
-  final public static function setMultipleConnectionInfo(array $databases) {
+  final public static function setMultipleConnectionInfo(array $databases, $class_loader = NULL, $app_root = NULL) {
     foreach ($databases as $key => $targets) {
       foreach ($targets as $target => $info) {
-        self::addConnectionInfo($key, $target, $info);
+        self::addConnectionInfo($key, $target, $info, $class_loader, $app_root);
       }
     }
   }
diff --git a/core/lib/Drupal/Core/Site/Settings.php b/core/lib/Drupal/Core/Site/Settings.php
index 245bc3537e..aedae4c951 100644
--- a/core/lib/Drupal/Core/Site/Settings.php
+++ b/core/lib/Drupal/Core/Site/Settings.php
@@ -143,57 +143,7 @@ public static function initialize($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'] . '\\', $app_root . '/' . $info['autoload']);
-        }
-      }
-    }
+    Database::setMultipleConnectionInfo($databases, $class_loader, $app_root);
 
     // Initialize Settings.
     new Settings($settings);
diff --git a/core/tests/Drupal/FunctionalTests/ExistingDrupal8StyleDatabaseConnectionInSettingsPhpTest.php b/core/tests/Drupal/FunctionalTests/ExistingDrupal8StyleDatabaseConnectionInSettingsPhpTest.php
index 855387f695..3a8a1342d8 100644
--- a/core/tests/Drupal/FunctionalTests/ExistingDrupal8StyleDatabaseConnectionInSettingsPhpTest.php
+++ b/core/tests/Drupal/FunctionalTests/ExistingDrupal8StyleDatabaseConnectionInSettingsPhpTest.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\FunctionalTests;
 
+use Drupal\Core\Database\Connection;
 use Drupal\Core\Database\Database;
 use Drupal\Tests\BrowserTestBase;
 
@@ -35,6 +36,19 @@ protected function setUp(): void {
     $namespace_search = "'namespace' => 'Drupal\\\\$driver\\\\Driver\\\\Database\\\\$driver',";
     $namespace_replace = "'namespace' => 'Drupal\\\\Core\\\\Database\\\\Driver\\\\$driver',";
     $contents = str_replace($namespace_search, $namespace_replace, $contents);
+
+    // Add a replica connection to the database settings.
+    $contents .= "\$databases['default']['replica'][] = array (\n";
+    $contents .= "  'database' => 'db',\n";
+    $contents .= "  'username' => 'db',\n";
+    $contents .= "  'password' => 'db',\n";
+    $contents .= "  'prefix' => 'test22806835',\n";
+    $contents .= "  'host' => 'db',\n";
+    $contents .= "  'port' => 3306,\n";
+    $contents .= "  $namespace_replace\n";
+    $contents .= "  'driver' => 'mysql',\n";
+    $contents .= ");\n";
+
     file_put_contents($filename, $contents);
   }
 
@@ -56,4 +70,14 @@ public function testExistingDrupal8StyleDatabaseConnectionInSettingsPhp() {
     $this->assertStringNotContainsString("'autoload' => 'core/modules/$driver/src/Driver/Database/$driver/", $contents);
   }
 
+  /**
+   * Confirms that the replica database connection works.
+   */
+  public function testReplicaDrupal8StyleDatabaseConnectionInSettingsPhp() {
+    $this->drupalLogin($this->drupalCreateUser());
+
+    $replica = Database::getConnection('replica', 'default');
+    $this->assertInstanceOf(Connection::class, $replica);
+  }
+
 }