--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -291,7 +291,8 @@ protected function theme($hook, $variables = array()) {
         // Set a variable for the 'theme_hook_suggestion'. This is used to
         // maintain backwards compatibility with template engines.
         $theme_hook_suggestion = $hook;
-        $info['preprocess functions'] = $base_hook_info['preprocess functions'];
+        $info['preprocess functions'] = array_merge($base_hook_info['preprocess functions'], (array) $info['preprocess functions']);
+        $info['preprocess functions'] = array_unique($info['preprocess functions']);
       }
     }
     if (isset($info['preprocess functions'])) {
diff --git a/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
index 0cd4e4e..bb6c822 100644
--- a/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -287,4 +287,19 @@ function testRegionClass() {
     $this->assertEqual(count($elements), 1, 'New class found.');
   }
 
+  /**
+   * Ensures suggestion preprocess functions run even for default implementations.
+   *
+   * The theme hook used by this test has its base preprocess function in a
+   * separate file, so this test also ensures that that file is correctly loaded
+   * when needed.
+   */
+  function testSuggestionPreprocessForDefaults() {
+    // Test with both an unprimed and primed theme registry.
+    drupal_theme_rebuild();
+    for ($i = 0; $i < 2; $i++) {
+      $this->drupalGet('theme-test/preprocess-suggestions');
+      $this->assertText('Theme hook implementor=test_theme_theme_test_preprocess__suggestion(). Foo=template_preprocess_theme_test_preprocess', 'Theme hook ran with data available from a preprocess function for the suggested hook.');
+    }
+  }
 }
diff --git a/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
index 335ed83..3e805f3 100644
--- a/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
+++ b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
@@ -143,4 +143,12 @@ public function nonHtml() {
     return new JsonResponse(['theme_initialized' => $theme_initialized]);
   }
 
+  /**
+   * Menu callback for testing preprocess functions are being run for theme
+   * suggestions.
+   */
+  function preprocessSuggestions() {
+    return array('#theme' => 'theme_test_preprocess_suggestions');
+  }
+
 }
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig
new file mode 100644
index 0000000..db8a8a4
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig
@@ -0,0 +1 @@
+{{ foo }}
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 7ba5c2d..64d45e7 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -55,6 +55,9 @@ function theme_test_theme($existing, $type, $theme, $path) {
   $info['test_theme_not_existing_function'] = array(
     'function' => 'test_theme_not_existing_function',
   );
+  $items['theme_test_preprocess_suggestions'] = array(
+    'variables' => array('foo' => ''),
+  );
   return $items;
 }
 
@@ -90,6 +93,17 @@ function theme_theme_test_function_template_override($variables) {
 }
 
 /**
+ * Implements hook_theme_suggestions_HOOK().
+ */
+function theme_test_theme_suggestions_theme_test_preprocess_suggestions($variables) {
+  return array('theme_test_preprocess_suggestions__' . 'suggestion');
+}
+
+function theme_test_preprocess_theme_test_preprocess_suggestions(&$variables) {
+  $variables['foo'] =  'Theme hook implementor=theme_theme_test_preprocess_suggestions().';
+}
+
+/**
  * Prepares variables for test render element templates.
  *
  * Default template: theme-test-render-element.html.twig.
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
index 2dbd188..1ff61cf 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
@@ -103,3 +103,10 @@ theme_test.non_html:
     _controller: '\Drupal\theme_test\ThemeTestController::nonHtml'
   requirements:
     _access: 'TRUE'
+
+theme_test.preprocess_suggestions:
+  path: '/theme-test/preprocess-suggestions'
+  defaults:
+    _controller: '\Drupal\theme_test\ThemeTestController::preprocessSuggestions'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/themes/bartik/.bartik.theme.swp b/core/themes/bartik/.bartik.theme.swp
new file mode 100644
index 0000000..8d19dbe
Binary files /dev/null and b/core/themes/bartik/.bartik.theme.swp differ
diff --git a/core/themes/bartik/bartik.theme b/core/themes/bartik/bartik.theme
index 7148e84..afc942c 100644
--- a/core/themes/bartik/bartik.theme
+++ b/core/themes/bartik/bartik.theme
@@ -128,3 +128,11 @@ function _bartik_process_page(&$variables) {
     $variables['site_slogan'] = Xss::filterAdmin($site_config->get('slogan'));
   }
 }
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function bartik_preprocess_theme_test_preprocess_suggestions__suggestion(&$variables) {
+  $variables['foo'] = 'Theme hook implementor=test_theme_theme_test_preprocess__suggestion().';
+}
+
diff --git a/core/themes/bartik/templates/theme-test-preprocess-suggestions--suggestion.html.twig b/core/themes/bartik/templates/theme-test-preprocess-suggestions--suggestion.html.twig
new file mode 100644
index 0000000..db8a8a4
--- /dev/null
+++ b/core/themes/bartik/templates/theme-test-preprocess-suggestions--suggestion.html.twig
@@ -0,0 +1 @@
+{{ foo }}