diff --git a/google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java b/google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java
index 5f5ebbaee204..bf79813f74e7 100644
--- a/google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java
+++ b/google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java
@@ -65,6 +65,7 @@ class MessageDispatcher {
   @InternalApi static final Duration PENDING_ACKS_SEND_DELAY = Duration.ofMillis(100);
 
   private final Executor executor;
+  private final SequentialExecutorService sequentialExecutor;
   private final ScheduledExecutorService systemExecutor;
   private final ApiClock clock;
 
@@ -217,6 +218,7 @@ void sendAckOperations(
     jobLock = new ReentrantLock();
     messagesWaiter = new MessageWaiter();
     this.clock = clock;
+    this.sequentialExecutor = new SequentialExecutorService(executor);
   }
 
   public void start() {
@@ -401,46 +403,51 @@ public void processOutstandingBatches() {
           outstandingMessageBatches.poll();
           batchCallback = nextBatch.doneCallback;
         }
-      }
 
-      final PubsubMessage message = outstandingMessage.receivedMessage().getMessage();
-      final AckHandler ackHandler = outstandingMessage.ackHandler();
-      final SettableApiFuture<AckReply> response = SettableApiFuture.create();
-      final AckReplyConsumer consumer =
-          new AckReplyConsumer() {
-            @Override
-            public void ack() {
-              response.set(AckReply.ACK);
-            }
-
-            @Override
-            public void nack() {
-              response.set(AckReply.NACK);
-            }
-          };
-      ApiFutures.addCallback(response, ackHandler, MoreExecutors.directExecutor());
-      executor.execute(
-          new Runnable() {
-            @Override
-            public void run() {
-              try {
-                if (ackHandler
-                    .totalExpiration
-                    .plusSeconds(messageDeadlineSeconds.get())
-                    .isBefore(now())) {
-                  // Message expired while waiting. We don't extend these messages anymore,
-                  // so it was probably sent to someone else. Don't work on it.
-                  // Don't nack it either, because we'd be nacking someone else's message.
-                  ackHandler.forget();
-                  return;
-                }
+        final PubsubMessage message = outstandingMessage.receivedMessage().getMessage();
+        final AckHandler ackHandler = outstandingMessage.ackHandler();
+        final SettableApiFuture<AckReply> response = SettableApiFuture.create();
+        final AckReplyConsumer consumer =
+            new AckReplyConsumer() {
+              @Override
+              public void ack() {
+                response.set(AckReply.ACK);
+              }
 
-                receiver.receiveMessage(message, consumer);
-              } catch (Exception e) {
-                response.setException(e);
+              @Override
+              public void nack() {
+                response.set(AckReply.NACK);
               }
-            }
-          });
+            };
+        ApiFutures.addCallback(response, ackHandler, MoreExecutors.directExecutor());
+        Runnable deliverMessageTask =
+            new Runnable() {
+              @Override
+              public void run() {
+                try {
+                  if (ackHandler
+                      .totalExpiration
+                      .plusSeconds(messageDeadlineSeconds.get())
+                      .isBefore(now())) {
+                    // Message expired while waiting. We don't extend these messages anymore,
+                    // so it was probably sent to someone else. Don't work on it.
+                    // Don't nack it either, because we'd be nacking someone else's message.
+                    ackHandler.forget();
+                    return;
+                  }
+
+                  receiver.receiveMessage(message, consumer);
+                } catch (Exception e) {
+                  response.setException(e);
+                }
+              }
+            };
+        if (message.getOrderingKey().isEmpty()) {
+          executor.execute(deliverMessageTask);
+        } else {
+          sequentialExecutor.submit(message.getOrderingKey(), deliverMessageTask);
+        }
+      }
       if (batchDone) {
         batchCallback.run();
       }
diff --git a/google-cloud-clients/google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/v1/MessageDispatcherTest.java b/google-cloud-clients/google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/v1/MessageDispatcherTest.java
index 785368bb13cb..0f5fb93664b1 100644
--- a/google-cloud-clients/google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/v1/MessageDispatcherTest.java
+++ b/google-cloud-clients/google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/v1/MessageDispatcherTest.java
@@ -28,8 +28,10 @@
 import com.google.pubsub.v1.ReceivedMessage;
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.LinkedList;
 import java.util.List;
+import java.util.Map;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.ScheduledThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
@@ -53,6 +55,7 @@ public void run() {
 
   private MessageDispatcher dispatcher;
   private LinkedBlockingQueue<AckReplyConsumer> consumers;
+  private Map<String, List<ByteString>> messagesByOrderingKey;
   private List<String> sentAcks;
   private List<ModAckItem> sentModAcks;
   private FakeClock clock;
@@ -72,6 +75,7 @@ static ModAckItem of(String ackId, int seconds) {
   @Before
   public void setUp() {
     consumers = new LinkedBlockingQueue<>();
+    messagesByOrderingKey = new HashMap<>();
     sentAcks = new ArrayList<>();
     sentModAcks = new ArrayList<>();
 
@@ -79,6 +83,12 @@ public void setUp() {
         new MessageReceiver() {
           @Override
           public void receiveMessage(final PubsubMessage message, final AckReplyConsumer consumer) {
+            List<ByteString> messages = messagesByOrderingKey.get(message.getOrderingKey());
+            if (messages == null) {
+              messages = new ArrayList<>();
+              messagesByOrderingKey.put(message.getOrderingKey(), messages);
+            }
+            messages.add(message.getData());
             consumers.add(consumer);
           }
         };
@@ -205,4 +215,47 @@ public void testDeadlineAdjustment() throws Exception {
 
     assertThat(dispatcher.computeDeadlineSeconds()).isEqualTo(42);
   }
+
+  private ReceivedMessage newReceivedMessage(String ackId, String orderingKey, String data) {
+    return ReceivedMessage.newBuilder()
+        .setAckId(ackId)
+        .setMessage(
+            PubsubMessage.newBuilder()
+                .setOrderingKey(orderingKey)
+                .setData(ByteString.copyFromUtf8(data))
+                .build())
+        .build();
+  }
+
+  @Test
+  public void testOrderingKey() throws Exception {
+    // Create messages with "orderA".
+    ReceivedMessage message1 = newReceivedMessage("ackId1", "orderA", "m1");
+    ReceivedMessage message2 = newReceivedMessage("ackId2", "orderA", "m2");
+    // Create messages with "orderB".
+    ReceivedMessage message3 = newReceivedMessage("ackId3", "orderB", "m3");
+    ReceivedMessage message4 = newReceivedMessage("ackId4", "orderB", "m4");
+    ReceivedMessage message5 = newReceivedMessage("ackId5", "orderB", "m5");
+
+    dispatcher.processReceivedMessages(Collections.singletonList(message1), NOOP_RUNNABLE);
+    consumers.take().ack();
+    dispatcher.processReceivedMessages(Collections.singletonList(message2), NOOP_RUNNABLE);
+    consumers.take().ack();
+    dispatcher.processReceivedMessages(Collections.singletonList(message3), NOOP_RUNNABLE);
+    consumers.take().ack();
+    dispatcher.processReceivedMessages(Collections.singletonList(message4), NOOP_RUNNABLE);
+    consumers.take().ack();
+    dispatcher.processReceivedMessages(Collections.singletonList(message5), NOOP_RUNNABLE);
+    consumers.take().ack();
+
+    assertThat(messagesByOrderingKey.get("orderA"))
+        .containsExactly(ByteString.copyFromUtf8("m1"), ByteString.copyFromUtf8("m2"))
+        .inOrder();
+    assertThat(messagesByOrderingKey.get("orderB"))
+        .containsExactly(
+            ByteString.copyFromUtf8("m3"),
+            ByteString.copyFromUtf8("m4"),
+            ByteString.copyFromUtf8("m5"))
+        .inOrder();
+  }
 }