diff --git a/.changes/nextrelease/query-mode-header.json b/.changes/nextrelease/query-mode-header.json
new file mode 100644
index 0000000000..9416c37c07
--- /dev/null
+++ b/.changes/nextrelease/query-mode-header.json
@@ -0,0 +1,7 @@
+[
+  {
+    "type": "enhancement",
+    "category": "",
+    "description": "Adds opt-in header for query-compatible services"
+  }
+]
\ No newline at end of file
diff --git a/src/AwsClient.php b/src/AwsClient.php
index 67ea82f4b8..c14257cb9c 100644
--- a/src/AwsClient.php
+++ b/src/AwsClient.php
@@ -12,6 +12,7 @@
 use Aws\Exception\AwsException;
 use Aws\Signature\SignatureProvider;
 use GuzzleHttp\Psr7\Uri;
+use Psr\Http\Message\RequestInterface;
 
 /**
  * Default AWS client implementation
@@ -275,6 +276,7 @@ public function __construct(array $args)
 
         if (!is_null($this->api->getMetadata('awsQueryCompatible'))) {
             $this->addQueryCompatibleInputMiddleware($this->api);
+            $this->addQueryModeHeader();
         }
 
         if (isset($args['with_resolved'])) {
@@ -524,6 +526,20 @@ private function addQueryCompatibleInputMiddleware(Service $api)
             );
     }
 
+    private function addQueryModeHeader()
+    {
+        $list = $this->getHandlerList();
+        $list->appendBuild(
+            Middleware::mapRequest(function (RequestInterface $r) {
+                return $r->withHeader(
+                    'x-amzn-query-mode',
+                    true
+                );
+            }),
+            'x-amzn-query-mode-header'
+        );
+    }
+
     private function addInvocationId()
     {
         // Add invocation id to each request
diff --git a/tests/AwsClientTest.php b/tests/AwsClientTest.php
index 702843d3b1..bcdfc88841 100644
--- a/tests/AwsClientTest.php
+++ b/tests/AwsClientTest.php
@@ -33,6 +33,7 @@
 class AwsClientTest extends TestCase
 {
     use UsesServiceTrait;
+    use TestServiceTrait;
 
     private function getApiProvider()
     {
@@ -982,4 +983,17 @@ public function testClientParameterOverridesDefaultAccountIdEndpointModeBuiltIns
 
         self::assertEquals($expectedAccountIdEndpointMode, $builtIns['AWS::Auth::AccountIdEndpointMode']);
     }
+
+    public function testQueryModeHeaderAdded()
+    {
+        $service = $this->generateTestService('json', ['awsQueryCompatible' => true]);
+        $client = $this->generateTestClient($service);
+        $list = $client->getHandlerList();
+        $list->setHandler(new MockHandler([new Result()]));
+        $list->appendSign(Middleware::tap(function($cmd, $req) {
+           $this->assertTrue($req->hasHeader('x-amzn-query-mode'));
+           $this->assertEquals(true, $req->getHeaderLine('x-amzn-query-mode'));
+        }));
+        $client->TestOperation();
+    }
 }