diff --git a/core/lib/Drupal/Core/Form/ConfigFormBaseTrait.php b/core/lib/Drupal/Core/Form/ConfigFormBaseTrait.php
index 172a77b..3cabd9b 100644
--- a/core/lib/Drupal/Core/Form/ConfigFormBaseTrait.php
+++ b/core/lib/Drupal/Core/Form/ConfigFormBaseTrait.php
@@ -55,6 +55,11 @@ protected function config($name) {
     if (in_array($name, $this->getEditableConfigNames())) {
       // Get a mutable object from the factory.
       $config = $config_factory->getEditable($name);
+
+      // Show a message if this config was overridden in settings.php.
+      if (isset($GLOBALS['config'][$name])) {
+        drupal_set_message(t('This form may include elements whose values are overridden using settings.php. Editing those values will save changes but will only be used if the overrides do not apply.'), 'warning');
+      }
     }
     else {
       $config = $config_factory->get($name);
diff --git a/core/modules/config/src/Tests/ConfigFormOverrideTest.php b/core/modules/config/src/Tests/ConfigFormOverrideTest.php
index be1e9df..e618403 100644
--- a/core/modules/config/src/Tests/ConfigFormOverrideTest.php
+++ b/core/modules/config/src/Tests/ConfigFormOverrideTest.php
@@ -23,6 +23,10 @@ class ConfigFormOverrideTest extends WebTestBase {
   public function testFormsWithOverrides() {
     $this->drupalLogin($this->drupalCreateUser(array('access administration pages', 'administer site configuration')));
 
+    // Ensure warning is not present if no overrides exist.
+    $this->drupalGet('admin/config/system/site-information');
+    $this->assertNoText('This form may include elements whose values are overridden using settings.php. Editing those values will save changes but will only be used if the overrides do not apply.');
+
     $overridden_name = 'Site name global conf override';
 
     // Set up an override.
@@ -39,6 +43,9 @@ public function testFormsWithOverrides() {
     $elements = $this->xpath('//input[@name="site_name"]');
     $this->assertIdentical((string) $elements[0]['value'], 'Drupal');
 
+    // Ensure the warning is displayed.
+    $this->assertText('This form may include elements whose values are overridden using settings.php. Editing those values will save changes but will only be used if the overrides do not apply.');
+
     // Submit the form and ensure the site name is not changed.
     $edit = array(
       'site_name' => 'Custom site name',