diff --git a/akka-http-core/src/main/scala/akka/http/impl/engine/client/OutgoingConnectionBlueprint.scala b/akka-http-core/src/main/scala/akka/http/impl/engine/client/OutgoingConnectionBlueprint.scala
index 9a93592426..950ef41e14 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/engine/client/OutgoingConnectionBlueprint.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/engine/client/OutgoingConnectionBlueprint.scala
@@ -61,7 +61,7 @@ private[http] object OutgoingConnectionBlueprint {
     val requestRendering: Flow[HttpRequest, ByteString, Unit] = Flow[HttpRequest]
       .map(RequestRenderingContext(_, hostHeader))
       .via(Flow[RequestRenderingContext].map(requestRendererFactory.renderToSource).named("renderer"))
-      .flatten(FlattenStrategy.concat)
+      .flattenConcat()
 
     val methodBypass = Flow[HttpRequest].map(_.method)
 
diff --git a/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala b/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
index 7b8806b52b..ea12116193 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
@@ -108,7 +108,7 @@ private[http] object HttpServerBluePrint {
       Flow[ResponseRenderingContext]
         .via(Flow[ResponseRenderingContext].transform(() ⇒ new ErrorsTo500ResponseRecovery(log)).named("recover")) // FIXME: simplify after #16394 is closed
         .via(Flow[ResponseRenderingContext].transform(() ⇒ responseRendererFactory.newRenderer).named("renderer"))
-        .flatten(FlattenStrategy.concat)
+        .flattenConcat()
         .via(Flow[ResponseRenderingOutput].transform(() ⇒ errorLogger(log, "Outgoing response stream error")).named("errorLogger"))
 
     BidiFlow.fromGraph(FlowGraph.create(requestParsingFlow, rendererPipeline, oneHundredContinueSource)((_, _, _) ⇒ ()) { implicit b ⇒
diff --git a/akka-http-core/src/main/scala/akka/http/impl/engine/ws/MessageToFrameRenderer.scala b/akka-http-core/src/main/scala/akka/http/impl/engine/ws/MessageToFrameRenderer.scala
index 97887d5a7f..707db8a92b 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/engine/ws/MessageToFrameRenderer.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/engine/ws/MessageToFrameRenderer.scala
@@ -5,7 +5,7 @@
 package akka.http.impl.engine.ws
 
 import akka.util.ByteString
-import akka.stream.scaladsl.{ FlattenStrategy, Source, Flow }
+import akka.stream.scaladsl.{ Source, Flow }
 
 import Protocol.Opcode
 import akka.http.scaladsl.model.ws._
@@ -32,6 +32,6 @@ private[http] object MessageToFrameRenderer {
         case bm: BinaryMessage          ⇒ streamedFrames(Opcode.Binary, bm.dataStream)
         case TextMessage.Strict(text)   ⇒ strictFrames(Opcode.Text, ByteString(text, "UTF-8"))
         case tm: TextMessage            ⇒ streamedFrames(Opcode.Text, tm.textStream.transform(() ⇒ new Utf8Encoder))
-      }.flatten(FlattenStrategy.concat)
+      }.flattenConcat()
   }
 }
diff --git a/akka-http-core/src/main/scala/akka/http/impl/util/package.scala b/akka-http-core/src/main/scala/akka/http/impl/util/package.scala
index dc10cfea7f..c2acb0aae7 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/util/package.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/util/package.scala
@@ -11,7 +11,7 @@ import language.higherKinds
 import java.nio.charset.Charset
 import java.util.concurrent.atomic.AtomicInteger
 import com.typesafe.config.Config
-import akka.stream.scaladsl.{ FlattenStrategy, Flow, Source }
+import akka.stream.scaladsl.{ Flow, Source }
 import akka.stream.stage._
 import scala.concurrent.duration.Duration
 import scala.concurrent.{ Await, Future }
@@ -54,7 +54,7 @@ package object util {
           .filter(_._1.nonEmpty)
           .map { case (prefix, tail) ⇒ (prefix.head, tail) }
       }
-      .flatten(FlattenStrategy.concat)
+      .flattenConcat()
 
   private[http] def printEvent[T](marker: String): Flow[T, T, Unit] =
     Flow[T].transform(() ⇒ new PushPullStage[T, T] {
diff --git a/akka-http-core/src/main/scala/akka/http/scaladsl/model/Multipart.scala b/akka-http-core/src/main/scala/akka/http/scaladsl/model/Multipart.scala
index 530546a8e6..a58d8e13df 100644
--- a/akka-http-core/src/main/scala/akka/http/scaladsl/model/Multipart.scala
+++ b/akka-http-core/src/main/scala/akka/http/scaladsl/model/Multipart.scala
@@ -15,7 +15,7 @@ import scala.collection.immutable
 import scala.util.{ Failure, Success, Try }
 import akka.stream.Materializer
 import akka.stream.io.SynchronousFileSource
-import akka.stream.scaladsl.{ FlattenStrategy, Source }
+import akka.stream.scaladsl.{ Source }
 import akka.http.scaladsl.util.FastFuture
 import akka.http.scaladsl.model.headers._
 import akka.http.impl.engine.rendering.BodyPartRenderer
@@ -40,7 +40,7 @@ sealed trait Multipart {
     val chunks =
       parts
         .transform(() ⇒ BodyPartRenderer.streamed(boundary, charset.nioCharset, partHeadersSizeHint = 128, log))
-        .flatten(FlattenStrategy.concat)
+        .flattenConcat()
     HttpEntity.Chunked(mediaType withBoundary boundary, chunks)
   }
 }
diff --git a/akka-http-core/src/test/scala/akka/http/impl/engine/parsing/RequestParserSpec.scala b/akka-http-core/src/test/scala/akka/http/impl/engine/parsing/RequestParserSpec.scala
index 2640c995d0..ad2ed0f687 100644
--- a/akka-http-core/src/test/scala/akka/http/impl/engine/parsing/RequestParserSpec.scala
+++ b/akka-http-core/src/test/scala/akka/http/impl/engine/parsing/RequestParserSpec.scala
@@ -19,7 +19,7 @@ import akka.http.scaladsl.model.headers._
 import akka.http.scaladsl.util.FastFuture
 import akka.http.scaladsl.util.FastFuture._
 import akka.stream.ActorMaterializer
-import akka.stream.scaladsl.{ FlattenStrategy, _ }
+import akka.stream.scaladsl._
 import akka.util.ByteString
 import com.typesafe.config.{ Config, ConfigFactory }
 import org.scalatest.matchers.Matcher
@@ -539,7 +539,7 @@ class RequestParserSpec extends FreeSpec with Matchers with BeforeAndAfterAll {
             }
           }
         }
-        .flatten(FlattenStrategy.concat)
+        .flattenConcat()
         .map(strictEqualify)
         .grouped(100000).runWith(Sink.head)
         .awaitResult(awaitAtMost)
diff --git a/akka-stream-tck/src/test/scala/akka/stream/tck/FlattenTest.scala b/akka-stream-tck/src/test/scala/akka/stream/tck/FlattenTest.scala
index 15510414fd..1fa849f442 100644
--- a/akka-stream-tck/src/test/scala/akka/stream/tck/FlattenTest.scala
+++ b/akka-stream-tck/src/test/scala/akka/stream/tck/FlattenTest.scala
@@ -3,7 +3,6 @@
  */
 package akka.stream.tck
 
-import akka.stream.scaladsl.FlattenStrategy
 import akka.stream.scaladsl.Sink
 import akka.stream.scaladsl.Source
 import org.reactivestreams.Publisher
@@ -13,7 +12,7 @@ class FlattenTest extends AkkaPublisherVerification[Int] {
   def createPublisher(elements: Long): Publisher[Int] = {
     val s1 = Source(iterable(elements / 2))
     val s2 = Source(iterable((elements + 1) / 2))
-    Source(List(s1, s2)).flatten(FlattenStrategy.concat).runWith(Sink.publisher)
+    Source(List(s1, s2)).flattenConcat().runWith(Sink.publisher)
   }
 
 }
diff --git a/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java b/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java
index db6c3d33e4..4ad1b7cffc 100644
--- a/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java
@@ -484,7 +484,7 @@ public class FlowTest extends StreamTest {
     mainInputs.add(Source.from(input2));
 
     final Flow<Source<Integer, BoxedUnit>, List<Integer>, BoxedUnit> flow = Flow.<Source<Integer, BoxedUnit>>create().
-      flatten(akka.stream.javadsl.FlattenStrategy.<Integer, BoxedUnit> concat()).grouped(6);
+      <Integer>flattenConcat().grouped(6);
     Future<List<Integer>> future = Source.from(mainInputs).via(flow)
         .runWith(Sink.<List<Integer>>head(), materializer);
 
diff --git a/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java b/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java
index 1f70e19d3a..80b3c301a3 100644
--- a/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java
+++ b/akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java
@@ -351,7 +351,7 @@ public class SourceTest extends StreamTest {
     mainInputs.add(Source.from(input2));
 
     Future<List<Integer>> future = Source.from(mainInputs)
-      .flatten(akka.stream.javadsl.FlattenStrategy.<Integer, BoxedUnit>concat()).grouped(6)
+      .<Integer>flattenConcat().grouped(6)
       .runWith(Sink.<List<Integer>>head(), materializer);
 
     List<Integer> result = Await.result(future, probe.dilated(FiniteDuration.create(3, TimeUnit.SECONDS)));
diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowConcatAllSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowConcatAllSpec.scala
index 9aa23f3a8b..9789071563 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowConcatAllSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowConcatAllSpec.scala
@@ -31,7 +31,7 @@ class FlowConcatAllSpec extends AkkaSpec {
       val main = Source(List(s1, s2, s3, s4, s5))
 
       val subscriber = TestSubscriber.manualProbe[Int]()
-      main.flatten(FlattenStrategy.concat).to(Sink(subscriber)).run()
+      main.flattenConcat().to(Sink(subscriber)).run()
       val subscription = subscriber.expectSubscription()
       subscription.request(10)
       for (i ← 1 to 10)
@@ -42,7 +42,7 @@ class FlowConcatAllSpec extends AkkaSpec {
 
     "work together with SplitWhen" in {
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(1 to 10).splitWhen(_ % 2 == 0).flatten(FlattenStrategy.concat).runWith(Sink(subscriber))
+      Source(1 to 10).splitWhen(_ % 2 == 0).flattenConcat().runWith(Sink(subscriber))
       val subscription = subscriber.expectSubscription()
       subscription.request(10)
       for (i ← (1 to 10))
@@ -54,7 +54,7 @@ class FlowConcatAllSpec extends AkkaSpec {
     "on onError on master stream cancel the current open substream and signal error" in assertAllStagesStopped {
       val publisher = TestPublisher.manualProbe[Source[Int, _]]()
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(publisher).flatten(FlattenStrategy.concat).to(Sink(subscriber)).run()
+      Source(publisher).flattenConcat().to(Sink(subscriber)).run()
 
       val upstream = publisher.expectSubscription()
       val downstream = subscriber.expectSubscription()
@@ -74,7 +74,7 @@ class FlowConcatAllSpec extends AkkaSpec {
     "on onError on master stream cancel the currently opening substream and signal error" in assertAllStagesStopped {
       val publisher = TestPublisher.manualProbe[Source[Int, _]]()
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(publisher).flatten(FlattenStrategy.concat).to(Sink(subscriber)).run()
+      Source(publisher).flattenConcat().to(Sink(subscriber)).run()
 
       val upstream = publisher.expectSubscription()
       val downstream = subscriber.expectSubscription()
@@ -97,7 +97,7 @@ class FlowConcatAllSpec extends AkkaSpec {
     "on onError on open substream, cancel the master stream and signal error " in assertAllStagesStopped {
       val publisher = TestPublisher.manualProbe[Source[Int, _]]()
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(publisher).flatten(FlattenStrategy.concat).to(Sink(subscriber)).run()
+      Source(publisher).flattenConcat().to(Sink(subscriber)).run()
 
       val upstream = publisher.expectSubscription()
       val downstream = subscriber.expectSubscription()
@@ -117,7 +117,7 @@ class FlowConcatAllSpec extends AkkaSpec {
     "on cancellation cancel the current open substream and the master stream" in assertAllStagesStopped {
       val publisher = TestPublisher.manualProbe[Source[Int, _]]()
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(publisher).flatten(FlattenStrategy.concat).to(Sink(subscriber)).run()
+      Source(publisher).flattenConcat().to(Sink(subscriber)).run()
 
       val upstream = publisher.expectSubscription()
       val downstream = subscriber.expectSubscription()
@@ -138,7 +138,7 @@ class FlowConcatAllSpec extends AkkaSpec {
     "on cancellation cancel the currently opening substream and the master stream" in assertAllStagesStopped {
       val publisher = TestPublisher.manualProbe[Source[Int, _]]()
       val subscriber = TestSubscriber.manualProbe[Int]()
-      Source(publisher).flatten(FlattenStrategy.concat).to(Sink(subscriber)).run()
+      Source(publisher).flattenConcat().to(Sink(subscriber)).run()
 
       val upstream = publisher.expectSubscription()
       val downstream = subscriber.expectSubscription()
@@ -162,7 +162,7 @@ class FlowConcatAllSpec extends AkkaSpec {
       val up = TestPublisher.manualProbe[Source[Int, _]]()
       val down = TestSubscriber.manualProbe[Int]()
 
-      val flowSubscriber = Source.subscriber[Source[Int, _]].flatten(FlattenStrategy.concat).to(Sink(down)).run()
+      val flowSubscriber = Source.subscriber[Source[Int, _]].flattenConcat().to(Sink(down)).run()
 
       val downstream = down.expectSubscription()
       downstream.cancel()
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/FlattenStrategy.scala b/akka-stream/src/main/scala/akka/stream/javadsl/FlattenStrategy.scala
deleted file mode 100644
index 5bcd43588c..0000000000
--- a/akka-stream/src/main/scala/akka/stream/javadsl/FlattenStrategy.scala
+++ /dev/null
@@ -1,25 +0,0 @@
-/**
- * Copyright (C) 2014 Typesafe Inc. <http://www.typesafe.com>
- */
-package akka.stream.javadsl
-
-import akka.stream.scaladsl
-
-/**
- * Strategy that defines how a stream of streams should be flattened into a stream of simple elements.
- */
-abstract class FlattenStrategy[-S, T] extends scaladsl.FlattenStrategy[S, T]
-
-object FlattenStrategy {
-
-  /**
-   * Strategy that flattens a stream of streams by concatenating them. This means taking an incoming stream
-   * emitting its elements directly to the output until it completes and then taking the next stream. This has the
-   * consequence that if one of the input stream is infinite, no other streams after that will be consumed from.
-   */
-  def concat[T, U]: FlattenStrategy[Source[T, U], T] = Concat.asInstanceOf[FlattenStrategy[Source[T, U], T]]
-  /**
-   * INTERNAL API
-   */
-  private[akka] final case object Concat extends FlattenStrategy[Any, Nothing]
-}
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
index 1481f1a291..701d05e91b 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
@@ -839,9 +839,8 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
 
   /**
    * Transforms a stream of streams into a contiguous stream of elements using the provided flattening strategy.
-   * This operation can be used on a stream of element type [[Source]].
-   *
-   * '''Emits when''' (Concat) the current consumed substream has an element available
+   * This operation can be used on a stream of element type `Source[U]`.
+   * '''Emits when''' a currently consumed substream has an element available
    *
    * '''Backpressures when''' downstream backpressures
    *
@@ -850,8 +849,8 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * '''Cancels when''' downstream cancels
    *
    */
-  def flatten[U](strategy: FlattenStrategy[Out, U]): javadsl.Flow[In, U, Mat] =
-    new Flow(delegate.flatten(strategy))
+  def flattenConcat[U](): javadsl.Flow[In, U, Mat] =
+    new Flow(delegate.flattenConcat[U]()(conforms[U].asInstanceOf[Out <:< scaladsl.Source[U, _]]))
 
   /**
    * Concatenate the given [[Source]] to this [[Flow]], meaning that once this
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
index dba1b1c144..924bcbd799 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
@@ -841,10 +841,10 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
 
   /**
    * Transforms a stream of streams into a contiguous stream of elements using the provided flattening strategy.
-   * This operation can be used on a stream of element type [[Source]].
+   * This operation can be used on a stream of element type `Source[U]`.
    */
-  def flatten[U](strategy: FlattenStrategy[Out, U]): javadsl.Source[U, Mat] =
-    new Source(delegate.flatten(strategy))
+  def flattenConcat[U](): javadsl.Source[U, Mat] =
+    new Source(delegate.flattenConcat[U]()(conforms[U].asInstanceOf[Out <:< scaladsl.Source[U, _]]))
 
   /**
    * If the first element has not passed through this stage before the provided timeout, the stream is failed
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/FlattenStrategy.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/FlattenStrategy.scala
deleted file mode 100644
index 8ff8a1ca67..0000000000
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/FlattenStrategy.scala
+++ /dev/null
@@ -1,21 +0,0 @@
-/**
- * Copyright (C) 2009-2014 Typesafe Inc. <http://www.typesafe.com>
- */
-package akka.stream.scaladsl
-
-/**
- * Strategy that defines how a stream of streams should be flattened into a stream of simple elements.
- */
-abstract class FlattenStrategy[-S, +T]
-
-object FlattenStrategy {
-
-  /**
-   * Strategy that flattens a stream of streams by concatenating them. This means taking an incoming stream
-   * emitting its elements directly to the output until it completes and then taking the next stream. This has the
-   * consequence that if one of the input stream is infinite, no other streams after that will be consumed from.
-   */
-  def concat[T]: FlattenStrategy[Source[T, Any], T] = Concat.asInstanceOf[FlattenStrategy[Source[T, Any], T]]
-
-  private[akka] final case object Concat extends FlattenStrategy[Any, Nothing]
-}
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
index 64babe0375..9fe04c863e 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
@@ -1000,10 +1000,9 @@ trait FlowOps[+Out, +Mat] {
   }
 
   /**
-   * Transforms a stream of streams into a contiguous stream of elements using the provided flattening strategy.
-   * This operation can be used on a stream of element type [[akka.stream.scaladsl.Source]].
+   * Flattens a stream of [[Source]]s into a contiguous stream by fully consuming one stream after the other.
    *
-   * '''Emits when''' (Concat) the current consumed substream has an element available
+   * '''Emits when''' a currently consumed substream has an element available
    *
    * '''Backpressures when''' downstream backpressures
    *
@@ -1012,11 +1011,7 @@ trait FlowOps[+Out, +Mat] {
    * '''Cancels when''' downstream cancels
    *
    */
-  def flatten[U](strategy: FlattenStrategy[Out, U]): Repr[U, Mat] = strategy match {
-    case scaladsl.FlattenStrategy.Concat | javadsl.FlattenStrategy.Concat ⇒ deprecatedAndThen(ConcatAll())
-    case _ ⇒
-      throw new IllegalArgumentException(s"Unsupported flattening strategy [${strategy.getClass.getName}]")
-  }
+  def flattenConcat[U]()(implicit ev: Out <:< Source[U, _]): Repr[U, Mat] = deprecatedAndThen(ConcatAll())
 
   /**
    * If the first element has not passed through this stage before the provided timeout, the stream is failed