diff --git a/utils/src/main/java/software/amazon/awssdk/utils/async/AdditionalDataSubscriber.java b/utils/src/main/java/software/amazon/awssdk/utils/async/AdditionalDataSubscriber.java
new file mode 100644
index 000000000000..41cca81a9a18
--- /dev/null
+++ b/utils/src/main/java/software/amazon/awssdk/utils/async/AdditionalDataSubscriber.java
@@ -0,0 +1,142 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *
+ *  http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed
+ * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package software.amazon.awssdk.utils.async;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Supplier;
+import org.reactivestreams.Subscriber;
+import org.reactivestreams.Subscription;
+import software.amazon.awssdk.annotations.SdkProtectedApi;
+import software.amazon.awssdk.utils.Logger;
+import software.amazon.awssdk.utils.Validate;
+
+/**
+ * Allows additional data to be sent before invoking onComplete on the downstream subscriber.
+ */
+@SdkProtectedApi
+public class AdditionalDataSubscriber<T> extends DelegatingSubscriber<T, T> {
+    private static final Logger log = Logger.loggerFor(AdditionalDataSubscriber.class);
+
+    /**
+     * The subscription to the upstream subscriber.
+     */
+    private Subscription upstreamSubscription;
+
+    /**
+     * The amount of unfulfilled demand the downstream subscriber has opened against us.
+     */
+    private final AtomicLong downstreamDemand = new AtomicLong(0);
+
+    /**
+     * Whether the upstream subscriber has called onComplete on us.
+     */
+    private volatile boolean onCompleteCalledByUpstream = false;
+
+    /**
+     * Whether the upstream subscriber has called onError on us.
+     */
+    private volatile boolean onErrorCalledByUpstream = false;
+
+    /**
+     * Whether we have called onComplete on the downstream subscriber.
+     */
+    private AtomicBoolean onCompleteCalledOnDownstream = new AtomicBoolean(false);
+
+    private final Supplier<T> additionalDataSupplier;
+
+    public AdditionalDataSubscriber(Subscriber<? super T> subscriber, Supplier<T> additionalDataSupplier) {
+        super(Validate.paramNotNull(subscriber, "subscriber"));
+        this.additionalDataSupplier = Validate.paramNotNull(additionalDataSupplier, "additionalDataSupplier");
+    }
+
+    @Override
+    public void onSubscribe(Subscription subscription) {
+
+        if (upstreamSubscription != null) {
+            log.warn(() -> "Received duplicate subscription, cancelling the duplicate.", new IllegalStateException());
+            subscription.cancel();
+            return;
+        }
+
+        upstreamSubscription = subscription;
+
+        subscriber.onSubscribe(new Subscription() {
+
+            @Override
+            public void request(long l) {
+                if (onErrorCalledByUpstream) {
+                    return;
+                }
+
+                if (onCompleteCalledByUpstream) {
+                    sendAdditionalDataAndComplete();
+                    return;
+                }
+
+                addDownstreamDemand(l);
+                upstreamSubscription.request(l);
+            }
+
+            @Override
+            public void cancel() {
+                upstreamSubscription.cancel();
+            }
+        });
+    }
+
+    @Override
+    public void onError(Throwable throwable) {
+        onErrorCalledByUpstream = true;
+        subscriber.onError(throwable);
+    }
+
+    @Override
+    public void onNext(T t) {
+        Validate.paramNotNull(t, "item");
+        downstreamDemand.decrementAndGet();
+        subscriber.onNext(t);
+    }
+
+    @Override
+    public void onComplete() {
+        onCompleteCalledByUpstream = true;
+        if (downstreamDemand.get() > 0) {
+            sendAdditionalDataAndComplete();
+        }
+    }
+
+    private void addDownstreamDemand(long l) {
+
+        if (l > 0) {
+            downstreamDemand.getAndUpdate(current -> {
+                long newValue = current + l;
+                return newValue >= 0 ? newValue : Long.MAX_VALUE;
+            });
+        } else {
+            upstreamSubscription.cancel();
+            onError(new IllegalArgumentException("Demand must not be negative"));
+        }
+    }
+
+    private void sendAdditionalDataAndComplete() {
+        if (onCompleteCalledOnDownstream.compareAndSet(false, true)) {
+            T additionalData = additionalDataSupplier.get();
+            subscriber.onNext(additionalData);
+            subscriber.onComplete();
+        }
+    }
+}
diff --git a/utils/src/test/java/software/amazon/awssdk/utils/async/AdditionalDataSubscriberTckTest.java b/utils/src/test/java/software/amazon/awssdk/utils/async/AdditionalDataSubscriberTckTest.java
new file mode 100644
index 000000000000..33d343494aca
--- /dev/null
+++ b/utils/src/test/java/software/amazon/awssdk/utils/async/AdditionalDataSubscriberTckTest.java
@@ -0,0 +1,74 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *
+ *  http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed
+ * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package software.amazon.awssdk.utils.async;
+
+import java.util.concurrent.CompletableFuture;
+import org.reactivestreams.Subscriber;
+import org.reactivestreams.Subscription;
+import org.reactivestreams.tck.SubscriberWhiteboxVerification;
+import org.reactivestreams.tck.TestEnvironment;
+
+public class AdditionalDataSubscriberTckTest extends SubscriberWhiteboxVerification<Integer>  {
+    protected AdditionalDataSubscriberTckTest() {
+        super(new TestEnvironment());
+    }
+
+    @Override
+    public Subscriber<Integer> createSubscriber(WhiteboxSubscriberProbe<Integer> probe) {
+        Subscriber<Integer> foo = new SequentialSubscriber<>(s -> {}, new CompletableFuture<>());
+
+        return new AdditionalDataSubscriber<Integer>(foo, () -> Integer.MIN_VALUE) {
+            @Override
+            public void onError(Throwable throwable) {
+                super.onError(throwable);
+                probe.registerOnError(throwable);
+            }
+
+            @Override
+            public void onSubscribe(Subscription subscription) {
+                super.onSubscribe(subscription);
+                probe.registerOnSubscribe(new SubscriberPuppet() {
+                    @Override
+                    public void triggerRequest(long elements) {
+                        subscription.request(elements);
+                    }
+
+                    @Override
+                    public void signalCancel() {
+                        subscription.cancel();
+                    }
+                });
+            }
+
+            @Override
+            public void onNext(Integer nextItem) {
+                super.onNext(nextItem);
+                probe.registerOnNext(nextItem);
+            }
+
+            @Override
+            public void onComplete() {
+                super.onComplete();
+                probe.registerOnComplete();
+            }
+        };
+    }
+
+    @Override
+    public Integer createElement(int i) {
+        return i;
+    }
+}
diff --git a/utils/src/test/java/software/amazon/awssdk/utils/async/AdditionalDataSubscriberTest.java b/utils/src/test/java/software/amazon/awssdk/utils/async/AdditionalDataSubscriberTest.java
new file mode 100644
index 000000000000..7d577823c372
--- /dev/null
+++ b/utils/src/test/java/software/amazon/awssdk/utils/async/AdditionalDataSubscriberTest.java
@@ -0,0 +1,65 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *
+ *  http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed
+ * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package software.amazon.awssdk.utils.async;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import org.junit.jupiter.api.Test;
+import org.reactivestreams.Subscriber;
+
+public class AdditionalDataSubscriberTest {
+
+    @Test
+    void additionalDataSupplierNull_shouldThrowException() {
+        SequentialSubscriber<Integer> downstreamSubscriber = new SequentialSubscriber<Integer>(i -> {}, new CompletableFuture());
+        assertThatThrownBy(() -> new AdditionalDataSubscriber<>(downstreamSubscriber, null))
+            .hasMessageContaining("must not be null");
+    }
+
+    @Test
+    void subscriberNull_shouldThrowException() {
+        assertThatThrownBy(() -> new AdditionalDataSubscriber<>(null, () -> 1))
+            .hasMessageContaining("must not be null");
+    }
+
+    @Test
+    void additionalDataNotNull_shouldNotSendAdditionalData() {
+        List<Integer> result = new ArrayList<>();
+        CompletableFuture future = new CompletableFuture();
+        SequentialSubscriber<Integer> downstreamSubscriber = new SequentialSubscriber<Integer>(i -> result.add(i), future);
+
+        Subscriber<Integer> subscriber = new AdditionalDataSubscriber<>(downstreamSubscriber, () -> Integer.MAX_VALUE);
+
+        publishData(subscriber);
+
+        future.join();
+
+        assertThat(result).containsExactly(0, 1, 2, Integer.MAX_VALUE);
+    }
+
+    private void publishData(Subscriber<Integer> subscriber) {
+        SimplePublisher<Integer> simplePublisher = new SimplePublisher<>();
+        simplePublisher.subscribe(subscriber);
+        for (int i = 0; i < 3; i++) {
+            simplePublisher.send(i);
+        }
+        simplePublisher.complete();
+    }
+}