diff --git a/core/lib/Drupal/Component/Annotation/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Component/Annotation/Plugin/Discovery/AnnotatedClassDiscovery.php
index edfce73..a073665 100644
--- a/core/lib/Drupal/Component/Annotation/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Component/Annotation/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -47,6 +47,13 @@ class AnnotatedClassDiscovery implements DiscoveryInterface {
   protected $annotationReader;
 
   /**
+   * Additional namespaces to be scanned for annotation classes.
+   *
+   * @var string[]
+   */
+  protected $annotationNamespaces = array();
+
+  /**
    * Constructs a new instance.
    *
    * @param string[] $plugin_namespaces
@@ -55,10 +62,13 @@ class AnnotatedClassDiscovery implements DiscoveryInterface {
    * @param string $plugin_definition_annotation_name
    *   (optional) The name of the annotation that contains the plugin definition.
    *   Defaults to 'Drupal\Component\Annotation\Plugin'.
+   * @param string[] $annotation_namespaces
+   *   (optional) Additional namespaces to be scanned for annotation classes.
    */
-  function __construct($plugin_namespaces = array(), $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
+  function __construct($plugin_namespaces = array(), $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin', array $annotation_namespaces = array()) {
     $this->pluginNamespaces = $plugin_namespaces;
     $this->pluginDefinitionAnnotationName = $plugin_definition_annotation_name;
+    $this->annotationNamespaces = $annotation_namespaces;
   }
 
   /**
@@ -74,6 +84,8 @@ protected function getAnnotationReader() {
       // Add the namespaces from the main plugin annotation, like @EntityType.
       $namespace = substr($this->pluginDefinitionAnnotationName, 0, strrpos($this->pluginDefinitionAnnotationName, '\\'));
       $this->annotationReader->addNamespace($namespace);
+      // Register additional namespaces to be scanned for annotations.
+      array_walk($this->annotationNamespaces, [$this->annotationReader, 'addNamespace']);
     }
     return $this->annotationReader;
   }
diff --git a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
index 5b7b86b..14e9b2a 100644
--- a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
+++ b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
@@ -110,6 +110,14 @@ class DefaultPluginManager extends PluginManagerBase implements PluginManagerInt
   protected $namespaces;
 
   /**
+   * Additional namespaces the annotation discovery mechanism should scan for
+   * annotation definitions.
+   *
+   * @var string[]
+   */
+  protected $annotationNamespaces = array();
+
+  /**
    * Creates the discovery object.
    *
    * @param string|bool $subdir
@@ -124,13 +132,16 @@ class DefaultPluginManager extends PluginManagerBase implements PluginManagerInt
    * @param string $plugin_definition_annotation_name
    *   (optional) The name of the annotation that contains the plugin definition.
    *   Defaults to 'Drupal\Component\Annotation\Plugin'.
+   * @param string[] $annotation_namespaces
+   *   (optional) Additional namespaces to scan for annotation definitions.
    */
-  public function __construct($subdir, \Traversable $namespaces, ModuleHandlerInterface $module_handler, $plugin_interface = NULL, $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
+  public function __construct($subdir, \Traversable $namespaces, ModuleHandlerInterface $module_handler, $plugin_interface = NULL, $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin', array $annotation_namespaces = array()) {
     $this->subdir = $subdir;
     $this->namespaces = $namespaces;
     $this->pluginDefinitionAnnotationName = $plugin_definition_annotation_name;
     $this->pluginInterface = $plugin_interface;
     $this->moduleHandler = $module_handler;
+    $this->annotationNamespaces = $annotation_namespaces;
   }
 
   /**
@@ -200,6 +211,18 @@ public function clearCachedDefinitions() {
   }
 
   /**
+   * Adds a namespace to be scanned for annotation definitions.
+   *
+   * @param string $namespace
+   *   The namespace to add.
+   */
+  public function addAnnotationNamespace($namespace) {
+    if (!in_array($namespace, $this->annotationNamespaces)) {
+      array_push($this->annotationNamespaces, $namespace);
+    }
+  }
+
+  /**
    * Returns the cached plugin definitions of the decorated discovery class.
    *
    * @return array|null
@@ -259,7 +282,6 @@ protected function cacheSet($cid, $data, $expire = Cache::PERMANENT, array $tags
     }
   }
 
-
   /**
    * Performs extra processing on plugin definitions.
    *
@@ -278,7 +300,7 @@ public function processDefinition(&$definition, $plugin_id) {
    */
   protected function getDiscovery() {
     if (!$this->discovery) {
-      $discovery = new AnnotatedClassDiscovery($this->subdir, $this->namespaces, $this->pluginDefinitionAnnotationName);
+      $discovery = new AnnotatedClassDiscovery($this->subdir, $this->namespaces, $this->pluginDefinitionAnnotationName, $this->annotationNamespaces);
       $this->discovery = new ContainerDerivativeDiscoveryDecorator($discovery);
     }
     return $this->discovery;
diff --git a/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
index 6bc3bbb..bd9043c 100644
--- a/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -52,8 +52,10 @@ class AnnotatedClassDiscovery extends ComponentAnnotatedClassDiscovery {
    * @param string $plugin_definition_annotation_name
    *   (optional) The name of the annotation that contains the plugin definition.
    *   Defaults to 'Drupal\Component\Annotation\Plugin'.
+   * @param string[] $annotation_namespaces
+   *   (optional) Additional namespaces to scan for annotation definitions.
    */
-  function __construct($subdir, \Traversable $root_namespaces, $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
+  function __construct($subdir, \Traversable $root_namespaces, $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin', array $annotation_namespaces = array()) {
     if ($subdir) {
       // Prepend a directory separator to $subdir,
       // if it does not already have one.
@@ -65,7 +67,7 @@ function __construct($subdir, \Traversable $root_namespaces, $plugin_definition_
     }
     $this->rootNamespacesIterator = $root_namespaces;
     $plugin_namespaces = array();
-    parent::__construct($plugin_namespaces, $plugin_definition_annotation_name);
+    parent::__construct($plugin_namespaces, $plugin_definition_annotation_name, $annotation_namespaces);
   }
 
   /**