diff --git a/stream/src/main/mima-filters/1.1.x.backwards.excludes/mark-subsource-and-subflow-final.backwards.excludes b/stream/src/main/mima-filters/1.1.x.backwards.excludes/mark-subsource-and-subflow-final.backwards.excludes
new file mode 100644
index 00000000000..fe27a278a55
--- /dev/null
+++ b/stream/src/main/mima-filters/1.1.x.backwards.excludes/mark-subsource-and-subflow-final.backwards.excludes
@@ -0,0 +1,3 @@
+# Mark SubSource and SubFlow final class
+ProblemFilters.exclude[FinalClassProblem]("org.apache.pekko.stream.javadsl.SubSource")
+ProblemFilters.exclude[FinalClassProblem]("org.apache.pekko.stream.javadsl.SubFlow")
diff --git a/stream/src/main/scala/org/apache/pekko/stream/javadsl/SubFlow.scala b/stream/src/main/scala/org/apache/pekko/stream/javadsl/SubFlow.scala
index b9c7d089ab2..821f9117a78 100755
--- a/stream/src/main/scala/org/apache/pekko/stream/javadsl/SubFlow.scala
+++ b/stream/src/main/scala/org/apache/pekko/stream/javadsl/SubFlow.scala
@@ -53,7 +53,7 @@ object SubFlow {
  * SubFlows cannot contribute to the super-flow’s materialized value since they
  * are materialized later, during the runtime of the flow graph processing.
  */
-class SubFlow[In, Out, Mat](
+final class SubFlow[In, Out, Mat](
     delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Flow[In, Out, Mat]#Repr, scaladsl.Sink[In, Mat]]) {
 
   /** Converts this Flow to its Scala DSL counterpart */
diff --git a/stream/src/main/scala/org/apache/pekko/stream/javadsl/SubSource.scala b/stream/src/main/scala/org/apache/pekko/stream/javadsl/SubSource.scala
index 0de91a896ad..7239d31397f 100755
--- a/stream/src/main/scala/org/apache/pekko/stream/javadsl/SubSource.scala
+++ b/stream/src/main/scala/org/apache/pekko/stream/javadsl/SubSource.scala
@@ -48,7 +48,7 @@ object SubSource {
  * SubFlows cannot contribute to the super-flow’s materialized value since they
  * are materialized later, during the runtime of the flow graph processing.
  */
-class SubSource[Out, Mat](
+final class SubSource[Out, Mat](
     delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Source[Out, Mat]#Repr, scaladsl.RunnableGraph[Mat]]) {
 
   /** Converts this Flow to its Scala DSL counterpart */
diff --git a/stream/src/main/scala/org/apache/pekko/stream/scaladsl/SubFlow.scala b/stream/src/main/scala/org/apache/pekko/stream/scaladsl/SubFlow.scala
index d587b66d3a3..68566851b4d 100644
--- a/stream/src/main/scala/org/apache/pekko/stream/scaladsl/SubFlow.scala
+++ b/stream/src/main/scala/org/apache/pekko/stream/scaladsl/SubFlow.scala
@@ -13,15 +13,20 @@
 
 package org.apache.pekko.stream.scaladsl
 
-import scala.annotation.unchecked.uncheckedVariance
+import org.apache.pekko
+import pekko.annotation.DoNotInherit
+import pekko.stream._
 
-import org.apache.pekko.stream._
+import scala.annotation.unchecked.uncheckedVariance
 
 /**
  * A “stream of streams” sub-flow of data elements, e.g. produced by `groupBy`.
  * SubFlows cannot contribute to the super-flow’s materialized value since they
  * are materialized later, during the runtime of the flow graph processing.
+ *
+ * Not for user extension
  */
+@DoNotInherit
 trait SubFlow[+Out, +Mat, +F[+_], C] extends FlowOps[Out, Mat] {
 
   override type Repr[+T] = SubFlow[T, Mat @uncheckedVariance, F @uncheckedVariance, C @uncheckedVariance]