diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index e8cff8b..08ac747 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1204,8 +1204,9 @@ function _template_preprocess_default_variables() {
     'logged_in' => FALSE,
   );
 
-  // Give modules a chance to alter the default template variables.
+  // Give modules and themes a chance to alter the default template variables.
   \Drupal::moduleHandler()->alter('template_preprocess_default_variables', $variables);
+  \Drupal::theme()->alter('template_preprocess_default_variables', $variables);
 
   // Tell all templates where they are located.
   $variables['directory'] = \Drupal::theme()->getActiveTheme()->getPath();
diff --git a/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
index 28d47386..ead7e56 100644
--- a/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -322,4 +322,17 @@ public function testSuggestionPreprocessForDefaults() {
     }
   }
 
+  /**
+   * Check that a theme can provide default template variables.
+   */
+  public function testDefaultTemplateVariables() {
+    $this->assertThemeOutput('theme_test_default_variables', [], 'Default drink: "water"' . "\n");
+    $this->assertNotEqual(\Drupal::theme()->getActiveTheme()->getName(), 'test_theme');
+    // Set the active theme and rebuild the regsitry.
+    \Drupal::theme()->setActiveTheme(\Drupal::service('theme.initialization')->initTheme('test_theme'));
+    \Drupal::service('theme.registry')->reset();
+    $this->assertEqual(\Drupal::theme()->getActiveTheme()->getName(), 'test_theme');
+    $this->assertThemeOutput('theme_test_default_variables', [], 'Default drink: "coffee"' . "\n");
+  }
+
 }
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-default-variables.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-default-variables.html.twig
new file mode 100644
index 0000000..864eaea
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-default-variables.html.twig
@@ -0,0 +1 @@
+Default drink: "{{ default_drink }}"
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 8a8841e..3ca51f5 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -61,6 +61,9 @@ function theme_test_theme($existing, $type, $theme, $path) {
       'bar' => '',
     ],
   ];
+  $items['theme_test_default_variables'] = [
+    'variables' => [],
+  ];
   return $items;
 }
 
@@ -207,3 +210,10 @@ function theme_test_theme_suggestions_node(array $variables) {
 
   return $suggestions;
 }
+
+/**
+ * Implements hook_template_preprocess_default_variables_alter().
+ */
+function theme_test_template_preprocess_default_variables_alter(array &$variables) {
+  $variables['default_drink'] = 'water';
+}
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index aa399b6..165c15f 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -154,3 +154,10 @@ function test_theme_preprocess_theme_test_preprocess_suggestions__kitten__flamin
 function test_theme_preprocess_theme_test_preprocess_suggestions__kitten__meerkat__tarsier__moose(&$variables) {
   $variables['bar'] = 'Moose';
 }
+
+/**
+ * Implements hook_template_preprocess_default_variables_alter().
+ */
+function test_theme_template_preprocess_default_variables_alter(array &$variables) {
+  $variables['default_drink'] = 'coffee';
+}