diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 7bfe57b..abc5652 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -1136,7 +1136,11 @@ function system_system_info_alter(&$info, Extension $file, $type) {
 }
 
 /**
- * Gets the name of the default region for a given theme.
+ * Gets the name of the default region for a given theme, which will be one of:
+ *   A region name specified by 'block_default_region' in the theme's info.yml
+ *   The name of the first region specified in the theme's info.yml.
+ *   An empty string '' if the theme has no regions.
+ *
  *
  * @param $theme
  *   The name of a theme.
@@ -1145,7 +1149,13 @@ function system_system_info_alter(&$info, Extension $file, $type) {
  */
 function system_default_region($theme) {
   $regions = array_keys(system_region_list($theme, REGIONS_VISIBLE));
-  return isset($regions[0]) ? $regions[0] : '';
+  $info = system_get_info('theme', $theme);
+  if (isset($info['block_default_region']) && in_array($info['block_default_region'], $regions)) {
+    return $info['block_default_region'];
+  }
+  else {
+    return isset($regions[0]) ? $regions[0] : '';
+  }
 }
 
 /**