diff --git a/core/lib/Drupal/Core/Cache/DatabaseBackend.php b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
index 69bbb922e5..da7334f914 100644
--- a/core/lib/Drupal/Core/Cache/DatabaseBackend.php
+++ b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
@@ -82,9 +82,6 @@ class DatabaseBackend implements CacheBackendInterface {
    *   table.
    * @param \Drupal\Component\Serialization\ObjectAwareSerializationInterface $serializer
    *   The serializer to use.
-   *
-   * @todo Make $serializer parameter mandatory in Drupal 9.x, in
-   *   https://www.drupal.org/project/drupal/issues/3014548.
    */
   public function __construct(Connection $connection, CacheTagsChecksumInterface $checksum_provider, $bin, $max_rows = NULL, ObjectAwareSerializationInterface $serializer = NULL) {
     // All cache tables should be prefixed with 'cache_'.
@@ -94,7 +91,13 @@ public function __construct(Connection $connection, CacheTagsChecksumInterface $
     $this->connection = $connection;
     $this->checksumProvider = $checksum_provider;
     $this->maxRows = $max_rows === NULL ? static::DEFAULT_MAX_ROWS : $max_rows;
-    $this->serializer = $serializer ?: \Drupal::service('serialization.phpserialize');
+    if ($serializer) {
+      $this->serializer = $serializer;
+    }
+    else {
+      @trigger_error('The serializer service will be a mandatory parameter in Drupal 9.0.x. See https://www.drupal.org/node/3014688.', E_USER_DEPRECATED);
+      $this->serializer = \Drupal::service('serialization.phpserialize');
+    }
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Cache/DatabaseBackendFactory.php b/core/lib/Drupal/Core/Cache/DatabaseBackendFactory.php
index 80a068a6e6..ec1639ecea 100644
--- a/core/lib/Drupal/Core/Cache/DatabaseBackendFactory.php
+++ b/core/lib/Drupal/Core/Cache/DatabaseBackendFactory.php
@@ -49,15 +49,18 @@ class DatabaseBackendFactory implements CacheFactoryInterface {
    *   The serializer to use.
    *
    * @throws \BadMethodCallException
-   *
-   * @todo Make $serializer parameter mandatory in Drupal 9.x, in
-   *   https://www.drupal.org/project/drupal/issues/3014548.
    */
   public function __construct(Connection $connection, CacheTagsChecksumInterface $checksum_provider, Settings $settings = NULL, ObjectAwareSerializationInterface $serializer = NULL) {
     $this->connection = $connection;
     $this->checksumProvider = $checksum_provider;
     $this->settings = $settings ?: Settings::getInstance();
-    $this->serializer = $serializer ?: \Drupal::service('serialization.phpserialize');
+    if ($serializer) {
+      $this->serializer = $serializer;
+    }
+    else {
+      @trigger_error('The serializer service will be a mandatory parameter in Drupal 9.0.x. See https://www.drupal.org/node/3014688.', E_USER_DEPRECATED);
+      $this->serializer = \Drupal::service('serialization.phpserialize');
+    }
   }
 
   /**
diff --git a/core/tests/Drupal/KernelTests/Core/Cache/DatabaseBackendTest.php b/core/tests/Drupal/KernelTests/Core/Cache/DatabaseBackendTest.php
index 35a7d4f982..393ceab562 100644
--- a/core/tests/Drupal/KernelTests/Core/Cache/DatabaseBackendTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Cache/DatabaseBackendTest.php
@@ -89,6 +89,16 @@ public function testGarbageCollection() {
     $this->assertFalse($backend->get('test0'));
   }
 
+  /**
+   * Tests the deprecation error triggering when the serializer param is missed.
+   *
+   * @group legacy
+   * @expectedDeprecation The serializer service will be a mandatory parameter in Drupal 9.0.x. See https://www.drupal.org/node/3014688.
+   */
+  public function testDeprecationErrorTriggering() {
+    new DatabaseBackend($this->container->get('database'), $this->container->get('cache_tags.invalidator.checksum'), 'default', static::$maxRows);
+  }
+
   /**
    * Gets the number of rows in the test cache bin database table.
    *
diff --git a/core/tests/Drupal/Tests/Core/Cache/DatabaseBackendFactoryTest.php b/core/tests/Drupal/Tests/Core/Cache/DatabaseBackendFactoryTest.php
index a720fc70ea..77823a66a7 100644
--- a/core/tests/Drupal/Tests/Core/Cache/DatabaseBackendFactoryTest.php
+++ b/core/tests/Drupal/Tests/Core/Cache/DatabaseBackendFactoryTest.php
@@ -7,8 +7,10 @@
 use Drupal\Core\Cache\DatabaseBackend;
 use Drupal\Core\Cache\DatabaseBackendFactory;
 use Drupal\Core\Database\Connection;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
 use Drupal\Core\Site\Settings;
 use Drupal\Tests\UnitTestCase;
+use Symfony\Component\DependencyInjection\Definition;
 
 /**
  * @coversDefaultClass \Drupal\Core\Cache\DatabaseBackendFactory
@@ -33,6 +35,23 @@ public function testGet(array $settings, $expected_max_rows_foo, $expected_max_r
     $this->assertSame($expected_max_rows_bar, $database_backend_factory->get('bar')->getMaxRows());
   }
 
+  /**
+   * Tests the deprecation error triggering when the serializer param is missed.
+   *
+   * @group legacy
+   * @expectedDeprecation The serializer service will be a mandatory parameter in Drupal 9.0.x. See https://www.drupal.org/node/3014688.
+   */
+  public function testDeprecationErrorTriggering() {
+    $container = new ContainerBuilder();
+    $container->setDefinition('serialization.phpserialize', new Definition(new PhpSerialize()));
+    \Drupal::setContainer($container);
+    new DatabaseBackendFactory(
+      $this->prophesize(Connection::class)->reveal(),
+      $this->prophesize(CacheTagsChecksumInterface::class)->reveal(),
+      new Settings([])
+    );
+  }
+
   public function getProvider() {
     return [
       'default' => [