diff --git a/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
index ef6027256c..3436ec7366 100644
--- a/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
@@ -220,10 +220,13 @@ public static function open(array &$connection_options = []) {
     // by one.
     $connection_options += [
       'init_commands' => [],
+      // @todo Switch to 'READ COMMITTED' for Drupal 10, see https://www.drupal.org/node/1650930
+      'isolation_level' => 'REPEATABLE READ',
     ];
 
     $connection_options['init_commands'] += [
       'sql_mode' => "SET sql_mode = 'ANSI,TRADITIONAL'",
+      'isolation' => 'SET SESSION TRANSACTION ISOLATION LEVEL ' . $connection_options['isolation_level'],
     ];
 
     // Execute initial commands.
diff --git a/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php b/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php
index dfac0158b7..2b5bf9f617 100644
--- a/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php
@@ -175,6 +175,12 @@ public function getFormOptions(array $database) {
       $form['advanced_options']['port']['#default_value'] = '3306';
     }
 
+    // Add the isolation_level option to settings.php.
+    $form['isolation_level'] = [
+      '#type' => 'value',
+      '#default_value' => empty($database['isolation_level']) ? 'READ COMMITTED' : $database['isolation_level'],
+    ];
+
     return $form;
   }
 
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
index 9da21be807..d7d1f678fa 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
@@ -301,6 +301,8 @@ protected function getCredentials() {
     $drivers = drupal_get_database_types();
     $form = $drivers[$driver]->getFormOptions($connection_options);
     $connection_options = array_intersect_key($connection_options, $form + $form['advanced_options']);
+    // Remove isolation_level since that options is not configurable in the UI.
+    unset($connection_options['isolation_level']);
     $edit = [
       $driver => $connection_options,
       'source_private_file_path' => $this->getSourceBasePath(),
diff --git a/core/tests/Drupal/FunctionalTests/Installer/InstallerIsolationLevelExistingDatabaseSettingsTest.php b/core/tests/Drupal/FunctionalTests/Installer/InstallerIsolationLevelExistingDatabaseSettingsTest.php
new file mode 100644
index 0000000000..cb46cb8cf8
--- /dev/null
+++ b/core/tests/Drupal/FunctionalTests/Installer/InstallerIsolationLevelExistingDatabaseSettingsTest.php
@@ -0,0 +1,49 @@
+<?php
+
+namespace Drupal\FunctionalTests\Installer;
+
+use Drupal\Core\Database\Database;
+
+/**
+ * Tests the isolation_level setting with existing database settings.
+ *
+ * @group Installer
+ */
+class InstallerIsolationLevelExistingDatabaseSettingsTest extends InstallerTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function prepareEnvironment() {
+    parent::prepareEnvironment();
+
+    $connection_info = Database::getConnectionInfo();
+    // The isolation_level option is only available for MySQL.
+    if ($connection_info['default']['driver'] !== 'mysql') {
+      $this->markTestSkipped("This test does not support the {$connection_info['default']['driver']} database driver.");
+    }
+
+    // Pre-configure database credentials in settings.php.
+    unset($connection_info['default']['pdo'], $connection_info['default']['init_commands']);
+    $connection_info['default']['isolation_level'] = 'REPEATABLE READ';
+
+    $this->settings['databases']['default'] = (object) [
+      'value' => $connection_info,
+      'required' => TRUE,
+    ];
+  }
+
+  /**
+   * Verifies that isolation_level is not overridden in the database settings.
+   */
+  public function testInstaller() {
+    $contents = file_get_contents($this->container->getParameter('app.root') . '/' . $this->siteDirectory . '/settings.php');
+    $this->assertStringContainsString("'isolation_level' => 'REPEATABLE READ',", $contents);
+  }
+
+}
diff --git a/core/tests/Drupal/FunctionalTests/Installer/InstallerIsolationLevelNoDatabaseSettingsTest.php b/core/tests/Drupal/FunctionalTests/Installer/InstallerIsolationLevelNoDatabaseSettingsTest.php
new file mode 100644
index 0000000000..5e66cd0a59
--- /dev/null
+++ b/core/tests/Drupal/FunctionalTests/Installer/InstallerIsolationLevelNoDatabaseSettingsTest.php
@@ -0,0 +1,40 @@
+<?php
+
+namespace Drupal\FunctionalTests\Installer;
+
+use Drupal\Core\Database\Database;
+
+/**
+ * Tests the isolation_level setting with no database settings.
+ *
+ * @group Installer
+ */
+class InstallerIsolationLevelNoDatabaseSettingsTest extends InstallerTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function prepareEnvironment() {
+    parent::prepareEnvironment();
+
+    // The isolation_level option is only available for MySQL.
+    $connection_info = Database::getConnectionInfo();
+    if ($connection_info['default']['driver'] !== 'mysql') {
+      $this->markTestSkipped("This test does not support the {$connection_info['default']['driver']} database driver.");
+    }
+  }
+
+  /**
+   * Verifies that the isolation_level was added to the database settings.
+   */
+  public function testInstaller() {
+    $contents = file_get_contents($this->container->getParameter('app.root') . '/' . $this->siteDirectory . '/settings.php');
+    $this->assertStringContainsString("'isolation_level' => 'READ COMMITTED',", $contents);
+  }
+
+}