diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index ecd2d50c5d..f955f97571 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -4,6 +4,7 @@ Yii Framework 2 Change Log
 2.0.16 under development
 ------------------------
 
+- Bug #16766: `yii\filters\ContentNegotiator` was not setting `Vary` header to inform cache recipients (koteq, cebe, samdark)
 - Bug #11960: Fixed `checked` option ignore in `yii\helpers\BaseHtml::checkbox()` (misantron)
 - Bug #14759: Fixed `yii\web\JsonResponseFormatter` output for `null` data (misantron)
 - Bug #16490: Fix schema on rbac init (marcelodeandrade)
diff --git a/framework/filters/ContentNegotiator.php b/framework/filters/ContentNegotiator.php
index a84b7280cd..84ca104ec5 100644
--- a/framework/filters/ContentNegotiator.php
+++ b/framework/filters/ContentNegotiator.php
@@ -154,9 +154,15 @@ class ContentNegotiator extends ActionFilter implements BootstrapInterface
         $request = $this->request ?: Yii::$app->getRequest();
         $response = $this->response ?: Yii::$app->getResponse();
         if (!empty($this->formats)) {
+            if (\count($this->formats) > 1) {
+                $response->getHeaders()->add('Vary', 'Accept');
+            }
             $this->negotiateContentType($request, $response);
         }
         if (!empty($this->languages)) {
+            if (\count($this->languages) > 1) {
+                $response->getHeaders()->add('Vary', 'Accept-Language');
+            }
             Yii::$app->language = $this->negotiateLanguage($request);
         }
     }
diff --git a/tests/framework/filters/ContentNegotiatorTest.php b/tests/framework/filters/ContentNegotiatorTest.php
index fae753baa0..c8ecc27929 100644
--- a/tests/framework/filters/ContentNegotiatorTest.php
+++ b/tests/framework/filters/ContentNegotiatorTest.php
@@ -76,4 +76,45 @@ class ContentNegotiatorTest extends TestCase
 
         $filter->beforeAction($action);
     }
+
+    public function testVaryHeader()
+    {
+        list($action, $filter) = $this->mockActionAndFilter();
+        $filter->formats = [];
+        $filter->languages = [];
+        $filter->beforeAction($action);
+        $this->assertFalse($filter->response->getHeaders()->has('Vary'));
+
+        list($action, $filter) = $this->mockActionAndFilter();
+        $filter->formats = ['application/json' => Response::FORMAT_JSON];
+        $filter->languages = ['en'];
+        $filter->beforeAction($action);
+        $this->assertFalse($filter->response->getHeaders()->has('Vary'));  // There is still nothing to vary
+
+        list($action, $filter) = $this->mockActionAndFilter();
+        $filter->formats = [
+            'application/json' => Response::FORMAT_JSON,
+            'application/xml' => Response::FORMAT_XML,
+        ];
+        $filter->languages = [];
+        $filter->beforeAction($action);
+        $this->assertContains('Accept', $filter->response->getHeaders()->get('Vary', [], false));
+
+        list($action, $filter) = $this->mockActionAndFilter();
+        $filter->formats = [];
+        $filter->languages = ['en', 'de'];
+        $filter->beforeAction($action);
+        $this->assertContains('Accept-Language', $filter->response->getHeaders()->get('Vary', [], false));
+
+        list($action, $filter) = $this->mockActionAndFilter();
+        $filter->formats = [
+            'application/json' => Response::FORMAT_JSON,
+            'application/xml' => Response::FORMAT_XML,
+        ];
+        $filter->languages = ['en', 'de'];
+        $filter->beforeAction($action);
+        $varyHeader = $filter->response->getHeaders()->get('Vary', [], false);
+        $this->assertContains('Accept', $varyHeader);
+        $this->assertContains('Accept-Language', $varyHeader);
+    }
 }