diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeSuggestionsAlterTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeSuggestionsAlterTest.php
new file mode 100644
index 0000000..39c2cec
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeSuggestionsAlterTest.php
@@ -0,0 +1,79 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Theme\ThemeSuggestionsAlterTest.
+ */
+
+namespace Drupal\system\Tests\Theme;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests theme suggestion alter hooks.
+ */
+class ThemeSuggestionsAlterTest extends WebTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('theme_test');
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Theme suggestions alter',
+      'description' => 'Test theme suggestion alter hooks.',
+      'group' => 'Theme',
+    );
+  }
+
+  function setUp() {
+    parent::setUp();
+    theme_enable(array('test_theme'));
+  }
+
+  /**
+   * Tests that theme suggestion alter hooks work for templates.
+   */
+  function testTemplateSuggestionsAlter() {
+    $this->drupalGet('theme-test/suggestion-alter');
+    $this->assertText('Original template.', 'Original template rendered.');
+
+    // Enable test_theme and test that themes can alter template suggestions.
+    config('system.theme')
+      ->set('default', 'test_theme')
+      ->save();
+    $this->drupalGet('theme-test/suggestion-alter');
+    $this->assertText('Template overridden based on new theme suggestion provided by the test_theme theme.', 'Template suggestion provided by theme rendered.');
+
+    // Enable the theme_suggestions_test module to test modules implementing
+    // suggestions alter hooks.
+    module_enable(array('theme_suggestions_test'));
+    $this->drupalGet('theme-test/suggestion-alter');
+    $this->assertText('Template overridden based on new theme suggestion provided by a module.', 'Template suggestion provided by module rendered.');
+  }
+
+  /**
+   * Tests that theme suggestion alter hooks work for theme functions.
+   */
+  function testThemeFunctionSuggestionsAlter() {
+    $this->drupalGet('theme-test/function-suggestion-alter');
+    $this->assertText('Original theme function.', 'Original theme function rendered.');
+
+    // Enable test_theme and test that themes can alter theme suggestions.
+    config('system.theme')
+      ->set('default', 'test_theme')
+      ->save();
+    $this->drupalGet('theme-test/function-suggestion-alter');
+    $this->assertText('Theme function overridden based on new theme suggestion provided by the test_theme theme.', 'Theme function suggestion provided by theme rendered.');
+
+    // Enable the theme_suggestions_test module to test modules implementing
+    // suggestions alter hooks.
+    module_enable(array('theme_suggestions_test'));
+    $this->drupalGet('theme-test/function-suggestion-alter');
+    $this->assertText('Theme function overridden based on new theme suggestion provided by a module.', 'Theme function suggestion provided by module rendered.');
+  }
+
+}
diff --git a/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.info.yml b/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.info.yml
new file mode 100644
index 0000000..95f429f
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.info.yml
@@ -0,0 +1,7 @@
+name: 'Theme suggestions test'
+type: module
+description: 'Support module for testing theme suggestions.'
+package: Testing
+version: VERSION
+core: 8.x
+hidden: true
diff --git a/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module b/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module
new file mode 100644
index 0000000..4b50763
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module
@@ -0,0 +1,20 @@
+<?php
+
+/**
+ * @file
+ * Support module for testing theme suggestions.
+ */
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function theme_suggestions_test_theme_suggestions_theme_test_suggestions_alter(array &$suggestions, array $variables) {
+  $suggestions[] = 'theme_test_suggestions__' . 'module_override';
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function theme_suggestions_test_theme_suggestions_theme_test_function_suggestions_alter(array &$suggestions, array $variables) {
+  $suggestions[] = 'theme_test_function_suggestions__' . 'module_override';
+}
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-suggestions.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-suggestions.html.twig
new file mode 100644
index 0000000..dfc848c
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-suggestions.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Original template.
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index ca8439d..e6968c7 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -14,6 +14,13 @@ function theme_test_theme($existing, $type, $theme, $path) {
   $items['theme_test_template_test_2'] = array(
     'template' => 'theme_test.template_test',
   );
+  $items['theme_test_suggestions'] = array(
+    'template' => 'theme-test-suggestions',
+    'variables' => array(),
+  );
+  $items['theme_test_function_suggestions'] = array(
+    'variables' => array(),
+  );
   $items['theme_test_foo'] = array(
     'variables' => array('foo' => NULL),
   );
@@ -49,6 +56,16 @@ function theme_test_menu() {
     'theme callback' => '_theme_custom_theme',
     'type' => MENU_CALLBACK,
   );
+  $items['theme-test/suggestion-alter'] = array(
+    'page callback' => 'theme_test_suggestion_alter',
+    'access callback' => TRUE,
+    'type' => MENU_CALLBACK,
+  );
+  $items['theme-test/function-suggestion-alter'] = array(
+    'page callback' => 'theme_test_function_suggestion_alter',
+    'access callback' => TRUE,
+    'type' => MENU_CALLBACK,
+  );
   $items['theme-test/alter'] = array(
     'title' => 'Suggestion',
     'page callback' => '_theme_test_alter',
@@ -178,3 +195,38 @@ function template_preprocess_theme_test_render_element(&$variables) {
 function theme_theme_test_render_element_children($variables) {
   return drupal_render($variables['element']);
 }
+
+/**
+ * Page callback: Displays theme suggestions test data.
+ *
+ * Used for testing theme hooks defined as template files.
+ *
+ * @return array
+ *   A render array to generate theme suggestions test output.
+ *
+ * @see theme_test_menu()
+ */
+function theme_test_suggestion_alter() {
+  return array('#theme' => 'theme_test_suggestions');
+}
+
+/**
+ * Page callback: Displays theme suggestions test data.
+ *
+ * Used for testing theme hooks defined as theme functions.
+ *
+ * @return array
+ *   A render array to generate theme suggestions test output.
+ *
+ * @see theme_test_menu()
+ */
+function theme_test_function_suggestion_alter() {
+  return array('#theme' => 'theme_test_function_suggestions');
+}
+
+/**
+ * Returns HTML for a theme function suggestion test.
+ */
+function theme_theme_test_function_suggestions($variables) {
+  return 'Original theme function.';
+}
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-suggestions--module-override.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-suggestions--module-override.html.twig
new file mode 100644
index 0000000..26ce57b
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-suggestions--module-override.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Template overridden based on new theme suggestion provided by a module.
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-suggestions--theme-override.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-suggestions--theme-override.html.twig
new file mode 100644
index 0000000..dee829f
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-suggestions--theme-override.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Template overridden based on new theme suggestion provided by the test_theme theme.
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index 62b5abf..9b10b2b 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -29,3 +29,43 @@ function test_theme_theme_test__suggestion($variables) {
 function test_theme_theme_test_alter_alter(&$data) {
   $data = 'test_theme_theme_test_alter_alter was invoked';
 }
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function test_theme_theme_suggestions_theme_test_suggestions_alter(array &$suggestions, array $variables) {
+  // Theme alter hooks run after module alter hooks, so add this theme
+  // suggestion to the beginning of the array so that the suggestion added by
+  // the theme_suggestions_test module can be picked up when that module is
+  // enabled.
+  array_unshift($suggestions, 'theme_test_suggestions__' . 'theme_override');
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function test_theme_theme_suggestions_theme_test_function_suggestions_alter(array &$suggestions, array $variables) {
+  // Theme alter hooks run after module alter hooks, so add this theme
+  // suggestion to the beginning of the array so that the suggestion added by
+  // the theme_suggestions_test module can be picked up when that module is
+  // enabled.
+  array_unshift($suggestions, 'theme_test_function_suggestions__' . 'theme_override');
+}
+
+/**
+ * Returns HTML for a theme function suggestion test.
+ *
+ * Implements the theme_test_function_suggestions__theme_override suggestion.
+ */
+function test_theme_theme_test_function_suggestions__theme_override($variables) {
+  return 'Theme function overridden based on new theme suggestion provided by the test_theme theme.';
+}
+
+/**
+ * Returns HTML for a theme function suggestion test.
+ *
+ * Implements the theme_test_function_suggestions__module_override suggestion.
+ */
+function test_theme_theme_test_function_suggestions__module_override($variables) {
+  return 'Theme function overridden based on new theme suggestion provided by a module.';
+}