diff --git a/stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/BoundedSourceQueueSpec.scala b/stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/BoundedSourceQueueSpec.scala
index 9989c816456..01bac3691a4 100644
--- a/stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/BoundedSourceQueueSpec.scala
+++ b/stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/BoundedSourceQueueSpec.scala
@@ -45,6 +45,7 @@ class BoundedSourceQueueSpec extends StreamSpec("""pekko.loglevel = debug
       }
 
       queue.complete()
+      assert(queue.isCompleted)
 
       val subIt = Iterator.continually(sub.requestNext())
       subIt.zip(elements.iterator).foreach {
@@ -81,6 +82,7 @@ class BoundedSourceQueueSpec extends StreamSpec("""pekko.loglevel = debug
       val queue =
         Source.queue[Int](1).toMat(Sink.fromSubscriber(sub))(Keep.left).run()
       queue.complete()
+      assert(queue.isCompleted)
       assertThrows[IllegalStateException](queue.complete())
     }
 
@@ -89,6 +91,7 @@ class BoundedSourceQueueSpec extends StreamSpec("""pekko.loglevel = debug
       val queue =
         Source.queue[Int](1).toMat(Sink.fromSubscriber(sub))(Keep.left).run()
       queue.fail(ex)
+      assert(queue.isCompleted)
       assertThrows[IllegalStateException](queue.fail(ex))
     }
 
@@ -98,6 +101,7 @@ class BoundedSourceQueueSpec extends StreamSpec("""pekko.loglevel = debug
         Source.queue[Int](10).toMat(Sink.fromSubscriber(sub))(Keep.left).run()
 
       queue.complete()
+      assert(queue.isCompleted)
       queue.offer(1) should be(QueueOfferResult.QueueClosed)
       sub.expectSubscriptionAndComplete()
     }
@@ -108,6 +112,7 @@ class BoundedSourceQueueSpec extends StreamSpec("""pekko.loglevel = debug
         Source.queue[Int](10).toMat(Sink.fromSubscriber(sub))(Keep.left).run()
 
       queue.fail(ex)
+      assert(queue.isCompleted)
       queue.offer(1) should be(QueueOfferResult.Failure(ex))
       sub.request(1)
       sub.expectError(ex)
@@ -180,6 +185,7 @@ class BoundedSourceQueueSpec extends StreamSpec("""pekko.loglevel = debug
       // where enqueueing an element concurrently with Done reaching the stage can lead to Enqueued being returned
       // but the element dropped (no guarantee of entering stream as documented in BoundedSourceQueue.offer
       queue.complete()
+      assert(queue.isCompleted)
 
       result.futureValue should be(counter.get())
     }
diff --git a/stream/src/main/scala/org/apache/pekko/stream/BoundedSourceQueue.scala b/stream/src/main/scala/org/apache/pekko/stream/BoundedSourceQueue.scala
index d5b2377e6d2..cf55dee581c 100644
--- a/stream/src/main/scala/org/apache/pekko/stream/BoundedSourceQueue.scala
+++ b/stream/src/main/scala/org/apache/pekko/stream/BoundedSourceQueue.scala
@@ -38,6 +38,13 @@ trait BoundedSourceQueue[T] {
    */
   def complete(): Unit
 
+  /**
+   * Returns true if the stream has been completed, either normally or with failure.
+   *
+   * @since 1.1.0
+   */
+  def isCompleted: Boolean
+
   /**
    * Completes the stream with a failure.
    */
diff --git a/stream/src/main/scala/org/apache/pekko/stream/impl/BoundedSourceQueue.scala b/stream/src/main/scala/org/apache/pekko/stream/impl/BoundedSourceQueue.scala
index a904145e8a3..5fd37481ea8 100644
--- a/stream/src/main/scala/org/apache/pekko/stream/impl/BoundedSourceQueue.scala
+++ b/stream/src/main/scala/org/apache/pekko/stream/impl/BoundedSourceQueue.scala
@@ -114,7 +114,7 @@ import pekko.stream.stage.{ GraphStageLogic, GraphStageWithMaterializedValue, Ou
     }
 
     object Mat extends BoundedSourceQueue[T] {
-      override def offer(elem: T): QueueOfferResult = state.get() match {
+      final override def offer(elem: T): QueueOfferResult = state.get() match {
         case Running | NeedsActivation =>
           if (queue.add(elem)) {
             // need to query state again because stage might have switched from Running -> NeedsActivation only after
@@ -130,21 +130,23 @@ import pekko.stream.stage.{ GraphStageLogic, GraphStageWithMaterializedValue, Ou
         case Done(result) => result
       }
 
-      override def complete(): Unit = {
+      final override def complete(): Unit = {
         if (state.get().isInstanceOf[Done])
           throw new IllegalStateException("The queue has already been completed.")
         if (setDone(Done(QueueOfferResult.QueueClosed)))
           Logic.callback.invoke(()) // if this thread won the completion race also schedule an async callback
       }
 
-      override def fail(ex: Throwable): Unit = {
+      final override def isCompleted: Boolean = state.get().isInstanceOf[Done]
+
+      final override def fail(ex: Throwable): Unit = {
         if (state.get().isInstanceOf[Done])
           throw new IllegalStateException("The queue has already been completed.")
         if (setDone(Done(QueueOfferResult.Failure(ex))))
           Logic.callback.invoke(()) // if this thread won the completion race also schedule an async callback
       }
 
-      override def size(): Int = queue.size()
+      final override def size(): Int = queue.size()
     }
 
     // some state transition helpers