diff --git a/core/assets/scaffold/files/default.settings.php b/core/assets/scaffold/files/default.settings.php
index 72be7750b9..d4782bbf03 100644
--- a/core/assets/scaffold/files/default.settings.php
+++ b/core/assets/scaffold/files/default.settings.php
@@ -138,6 +138,21 @@
  * request as needed.  The fourth line creates a new database with a name of
  * "extra".
  *
+ * For MySQL, MariaDB or equivalent databases can the option 'isolation_level'
+ * be set. The recommended option is 'READ COMMITTED'. Existing sites without
+ * this setting will default to 'REPEATABLE READ'. The other 2 options are
+ * 'READ UNCOMMITTED' and 'SERIALIZABLE'. They are available, but not supported.
+ * Use them at your own risk. For more info:
+ * https://dev.mysql.com/doc/refman/5.7/en/innodb-transaction-isolation-levels.html
+ *
+ * On your settings.php, change the isolation level:
+ * @code
+ * $databases['default']['default']['isolation_level'] = 'READ COMMITTED';
+ * $databases['default']['default']['isolation_level'] = 'REPEATABLE READ';
+ * $databases['default']['default']['isolation_level'] = 'READ UNCOMMITTED';
+ * $databases['default']['default']['isolation_level'] = 'SERIALIZABLE';
+ * @endcode
+ *
  * You can optionally set a prefix for all database table names by using the
  * 'prefix' setting. If a prefix is specified, the table name will be prepended
  * with its value. Be sure to use valid database characters only, usually
diff --git a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
index 244d0eeb46..7c4adb8ecc 100644
--- a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
+++ b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
@@ -519,6 +519,7 @@ protected function installParameters() {
     unset($connection_info['default']['autoload']);
     unset($connection_info['default']['pdo']);
     unset($connection_info['default']['init_commands']);
+    unset($connection_info['default']['isolation_level']);
     // Remove database connection info that is not used by SQLite.
     if ($driver === 'sqlite') {
       unset($connection_info['default']['username']);
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 1c61113ad3..705d1a3fb5 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/modules/migrate_drupal_ui/tests/src/Functional/d7/FilePathTest.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/FilePathTest.php
index 5f8652e8d2..8fd58676a5 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d7/FilePathTest.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/FilePathTest.php
@@ -127,6 +127,8 @@ public function testFilePath(string $file_private_path, string $file_public_path
     $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 => $connection_options,
       'version' => '7',
diff --git a/core/modules/mysql/src/Driver/Database/mysql/Connection.php b/core/modules/mysql/src/Driver/Database/mysql/Connection.php
index 4fbb2221ee..4cf4b93358 100644
--- a/core/modules/mysql/src/Driver/Database/mysql/Connection.php
+++ b/core/modules/mysql/src/Driver/Database/mysql/Connection.php
@@ -219,10 +219,25 @@ public static function open(array &$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',
     ];
 
+    $isolation_levels = [
+      'READ COMMITTED',
+      'REPEATABLE READ',
+      'READ UNCOMMITTED',
+      'SERIALIZABLE',
+    ];
+
+    if (!in_array($connection_options['isolation_level'], $isolation_levels)) {
+      throw new DatabaseTransactionIsolationLevelException($connection_options['isolation_level']);
+    }
+
     $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/modules/mysql/src/Driver/Database/mysql/DatabaseTransactionIsolationLevelException.php b/core/modules/mysql/src/Driver/Database/mysql/DatabaseTransactionIsolationLevelException.php
new file mode 100644
index 0000000000..97b73b6f0a
--- /dev/null
+++ b/core/modules/mysql/src/Driver/Database/mysql/DatabaseTransactionIsolationLevelException.php
@@ -0,0 +1,25 @@
+<?php
+
+namespace Drupal\mysql\Driver\Database\mysql;
+
+use Drupal\Core\Database\DatabaseException;
+
+/**
+ * Exception thrown if the isolation level is not valid.
+ *
+ * @see \Drupal\mysql\Driver\Database\mysql\Connection::open()
+ */
+class DatabaseTransactionIsolationLevelException extends \RuntimeException implements DatabaseException {
+
+  /**
+   * Constructs a DatabaseTransactionIsolationLevelException.
+   *
+   * @param string $isolation_level
+   *   The invalid isolation level.
+   */
+  public function __construct(string $isolation_level) {
+    $message = sprintf('The isolation level %s is not valid, use one of options available instead.', $isolation_level);
+    parent::__construct($message);
+  }
+
+}
diff --git a/core/modules/mysql/src/Driver/Database/mysql/Install/Tasks.php b/core/modules/mysql/src/Driver/Database/mysql/Install/Tasks.php
index a6ad642425..6d6b123cc6 100644
--- a/core/modules/mysql/src/Driver/Database/mysql/Install/Tasks.php
+++ b/core/modules/mysql/src/Driver/Database/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' => $database['isolation_level'] ?? 'READ COMMITTED',
+    ];
+
     return $form;
   }
 
diff --git a/core/modules/mysql/tests/src/Functional/InstallerIsolationLevelExistingSettingsTest.php b/core/modules/mysql/tests/src/Functional/InstallerIsolationLevelExistingSettingsTest.php
new file mode 100644
index 0000000000..796dcbf143
--- /dev/null
+++ b/core/modules/mysql/tests/src/Functional/InstallerIsolationLevelExistingSettingsTest.php
@@ -0,0 +1,69 @@
+<?php
+
+namespace Drupal\Tests\mysql\Functional;
+
+use Drupal\Core\Database\Database;
+use Drupal\FunctionalTests\Installer\InstallerExistingSettingsTest;
+
+/**
+ * Tests the isolation_level setting with existing database settings.
+ *
+ * @group Installer
+ */
+class InstallerIsolationLevelExistingSettingsTest extends InstallerExistingSettingsTest {
+
+  /**
+   * {@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.");
+    }
+  }
+
+  /**
+   * Verifies that isolation_level is not set in the database settings.
+   */
+  public function testInstaller() {
+    $contents = file_get_contents($this->container->getParameter('app.root') . '/' . $this->siteDirectory . '/settings.php');
+
+    // Test that isolation_level was not set.
+    $this->assertStringNotContainsString("'isolation_level' =>", $contents);
+
+    // Change the default database connection to use the isolation level from
+    // the test.
+    $connection_info = Database::getConnectionInfo();
+    $driver_test_connection = $connection_info['default'];
+    // We have asserted that the isolation level was not set.
+    unset($driver_test_connection['isolation_level']);
+    unset($driver_test_connection['init_commands']);
+
+    Database::renameConnection('default', 'original_database_connection');
+    Database::addConnectionInfo('default', 'default', $driver_test_connection);
+    // Close and reopen the database connection, so the database init commands
+    // get executed.
+    Database::closeConnection('default', 'default');
+    $connection = Database::getConnection('default', 'default');
+
+    $query = 'SELECT @@SESSION.tx_isolation';
+    // The database variable "tx_isolation" has been removed in MySQL v8.0 and
+    // has been replaced by "transaction_isolation".
+    // @see https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_tx_isolation
+    if (!$connection->isMariaDb() && version_compare($connection->version(), '8.0.0-AnyName', '>')) {
+      $query = 'SELECT @@SESSION.transaction_isolation';
+    }
+
+    // Test that transaction level is REPEATABLE-READ.
+    $this->assertEquals('REPEATABLE-READ', $connection->query($query)->fetchField());
+
+    // Restore the old database connection.
+    Database::addConnectionInfo('default', 'default', $connection_info['default']);
+    Database::closeConnection('default', 'default');
+    Database::getConnection('default', 'default');
+  }
+
+}
diff --git a/core/modules/mysql/tests/src/Functional/InstallerIsolationLevelNoDatabaseSettingsTest.php b/core/modules/mysql/tests/src/Functional/InstallerIsolationLevelNoDatabaseSettingsTest.php
new file mode 100644
index 0000000000..f54dca0595
--- /dev/null
+++ b/core/modules/mysql/tests/src/Functional/InstallerIsolationLevelNoDatabaseSettingsTest.php
@@ -0,0 +1,74 @@
+<?php
+
+namespace Drupal\Tests\mysql\Functional;
+
+use Drupal\Core\Database\Database;
+use Drupal\FunctionalTests\Installer\InstallerTestBase;
+
+/**
+ * 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');
+
+    // Test that isolation_level was set to "READ COMMITTED".
+    $this->assertStringContainsString("'isolation_level' => 'READ COMMITTED',", $contents);
+
+    // Change the default database connection to use the isolation level from
+    // the test.
+    $connection_info = Database::getConnectionInfo();
+    $driver_test_connection = $connection_info['default'];
+    // We have asserted that the isolation level was set to 'READ COMMITTED'.
+    $driver_test_connection['isolation_level'] = 'READ COMMITTED';
+    unset($driver_test_connection['init_commands']);
+
+    Database::renameConnection('default', 'original_database_connection');
+    Database::addConnectionInfo('default', 'default', $driver_test_connection);
+    // Close and reopen the database connection, so the database init commands
+    // get executed.
+    Database::closeConnection('default', 'default');
+    $connection = Database::getConnection('default', 'default');
+
+    $query = 'SELECT @@SESSION.tx_isolation';
+    // The database variable "tx_isolation" has been removed in MySQL v8.0 and
+    // has been replaced by "transaction_isolation".
+    // @see https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_tx_isolation
+    if (!$connection->isMariaDb() && version_compare($connection->version(), '8.0.0-AnyName', '>')) {
+      $query = 'SELECT @@SESSION.transaction_isolation';
+    }
+
+    // Test that transaction level is READ-COMMITTED.
+    $this->assertEquals('READ-COMMITTED', $connection->query($query)->fetchField());
+
+    // Restore the old database connection.
+    Database::addConnectionInfo('default', 'default', $connection_info['default']);
+    Database::closeConnection('default', 'default');
+    Database::getConnection('default', 'default');
+  }
+
+}
diff --git a/core/modules/mysql/tests/src/Kernel/ConnectionTest.php b/core/modules/mysql/tests/src/Kernel/ConnectionTest.php
new file mode 100644
index 0000000000..4b8b023fed
--- /dev/null
+++ b/core/modules/mysql/tests/src/Kernel/ConnectionTest.php
@@ -0,0 +1,39 @@
+<?php
+
+namespace Drupal\Tests\mysql\Kernel;
+
+use Drupal\Core\Database\Database;
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\mysql\Driver\Database\mysql\Connection;
+use Drupal\mysql\Driver\Database\mysql\DatabaseTransactionIsolationLevelException;
+
+/**
+ * Tests the connection class.
+ *
+ * @coversDefaultClass \Drupal\mysql\Driver\Database\mysql\Connection
+ *
+ * @group Mysql
+ */
+class ConnectionTest extends KernelTestBase {
+
+  /**
+   * Tests transaction isolation level when an invalid option is set.
+   *
+   * @covers ::open
+   */
+  public function testInvalidIsolationLevelOption() {
+    $connection_info = Database::getConnectionInfo();
+
+    // The isolation_level option is only available for MySQL.
+    if ($connection_info['default']['driver'] !== 'mysql') {
+      $this->markTestSkipped("This test does not support the {$connection_info['default']['driver']} database driver.");
+    }
+
+    $connection_info['default']['isolation_level'] = 'INVALID_LEVEL';
+    $this->expectException(DatabaseTransactionIsolationLevelException::class);
+    $this->expectExceptionMessage("The isolation level INVALID_LEVEL is not valid, use one of options available instead.");
+
+    Connection::open($connection_info['default']);
+  }
+
+}
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index 72be7750b9..d4782bbf03 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -138,6 +138,21 @@
  * request as needed.  The fourth line creates a new database with a name of
  * "extra".
  *
+ * For MySQL, MariaDB or equivalent databases can the option 'isolation_level'
+ * be set. The recommended option is 'READ COMMITTED'. Existing sites without
+ * this setting will default to 'REPEATABLE READ'. The other 2 options are
+ * 'READ UNCOMMITTED' and 'SERIALIZABLE'. They are available, but not supported.
+ * Use them at your own risk. For more info:
+ * https://dev.mysql.com/doc/refman/5.7/en/innodb-transaction-isolation-levels.html
+ *
+ * On your settings.php, change the isolation level:
+ * @code
+ * $databases['default']['default']['isolation_level'] = 'READ COMMITTED';
+ * $databases['default']['default']['isolation_level'] = 'REPEATABLE READ';
+ * $databases['default']['default']['isolation_level'] = 'READ UNCOMMITTED';
+ * $databases['default']['default']['isolation_level'] = 'SERIALIZABLE';
+ * @endcode
+ *
  * You can optionally set a prefix for all database table names by using the
  * 'prefix' setting. If a prefix is specified, the table name will be prepended
  * with its value. Be sure to use valid database characters only, usually