diff --git a/akka-http-core/src/main/scala/akka/http/impl/engine/rendering/BodyPartRenderer.scala b/akka-http-core/src/main/scala/akka/http/impl/engine/rendering/BodyPartRenderer.scala
index d0768924b9..752686ae49 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/engine/rendering/BodyPartRenderer.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/engine/rendering/BodyPartRenderer.scala
@@ -5,6 +5,8 @@
 package akka.http.impl.engine.rendering
 
 import java.nio.charset.Charset
+import akka.parboiled2.util.Base64
+
 import scala.collection.immutable
 import akka.event.LoggingAdapter
 import akka.http.scaladsl.model._
@@ -16,6 +18,8 @@ import akka.stream.stage._
 import akka.util.ByteString
 import HttpEntity._
 
+import scala.concurrent.forkjoin.ThreadLocalRandom
+
 /**
  * INTERNAL API
  */
@@ -110,4 +114,13 @@ private[http] object BodyPartRenderer {
 
     case x ⇒ r ~~ x ~~ CrLf
   }
+
+  /**
+   * Creates a new random number of the given length and base64 encodes it (using a custom "safe" alphabet).
+   */
+  def randomBoundary(length: Int = 18, random: java.util.Random = ThreadLocalRandom.current()): String = {
+    val array = new Array[Byte](length)
+    random.nextBytes(array)
+    Base64.custom.encodeToString(array, false)
+  }
 }
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 5cc1f17906..d036a992a6 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
@@ -4,15 +4,18 @@
 
 package akka.http.scaladsl.model
 
+import akka.event.{ NoLogging, LoggingAdapter }
+
 import scala.collection.immutable.VectorBuilder
 import scala.concurrent.duration.FiniteDuration
 import scala.concurrent.{ Future, ExecutionContext }
 import scala.collection.immutable
 import scala.util.{ Failure, Success, Try }
 import akka.stream.FlowMaterializer
-import akka.stream.scaladsl.Source
+import akka.stream.scaladsl.{ FlattenStrategy, Source }
 import akka.http.scaladsl.util.FastFuture
 import akka.http.scaladsl.model.headers._
+import akka.http.impl.engine.rendering.BodyPartRenderer
 import FastFuture._
 
 trait Multipart {
@@ -25,12 +28,29 @@ trait Multipart {
    * The Future is failed with an TimeoutException if one part isn't read completely after the given timeout.
    */
   def toStrict(timeout: FiniteDuration)(implicit ec: ExecutionContext, fm: FlowMaterializer): Future[Multipart.Strict]
+
+  /**
+   * Creates an entity from this multipart object.
+   */
+  def toEntity(charset: HttpCharset = HttpCharsets.`UTF-8`,
+               boundary: String = BodyPartRenderer.randomBoundary())(implicit log: LoggingAdapter = NoLogging): MessageEntity = {
+    val chunks =
+      parts
+        .transform(() ⇒ BodyPartRenderer.streamed(boundary, charset.nioCharset, partHeadersSizeHint = 128, log))
+        .flatten(FlattenStrategy.concat)
+    HttpEntity.Chunked(mediaType withBoundary boundary, chunks)
+  }
 }
 
 object Multipart {
 
   trait Strict extends Multipart {
     def strictParts: immutable.Seq[BodyPart.Strict]
+
+    override def toEntity(charset: HttpCharset, boundary: String)(implicit log: LoggingAdapter = NoLogging): HttpEntity.Strict = {
+      val data = BodyPartRenderer.strict(strictParts, boundary, charset.nioCharset, partHeadersSizeHint = 128, log)
+      HttpEntity(mediaType withBoundary boundary, data)
+    }
   }
 
   trait BodyPart {
diff --git a/akka-http/src/main/scala/akka/http/scaladsl/marshalling/MultipartMarshallers.scala b/akka-http/src/main/scala/akka/http/scaladsl/marshalling/MultipartMarshallers.scala
index 8649f12b92..b38c4afff9 100644
--- a/akka-http/src/main/scala/akka/http/scaladsl/marshalling/MultipartMarshallers.scala
+++ b/akka-http/src/main/scala/akka/http/scaladsl/marshalling/MultipartMarshallers.scala
@@ -5,41 +5,37 @@
 package akka.http.scaladsl.marshalling
 
 import scala.concurrent.forkjoin.ThreadLocalRandom
-import akka.parboiled2.util.Base64
 import akka.event.{ NoLogging, LoggingAdapter }
-import akka.stream.scaladsl.FlattenStrategy
 import akka.http.impl.engine.rendering.BodyPartRenderer
 import akka.http.scaladsl.model._
 
 trait MultipartMarshallers {
-  protected val multipartBoundaryRandom: java.util.Random = ThreadLocalRandom.current()
-
-  /**
-   * Creates a new random 144-bit number and base64 encodes it (using a custom "safe" alphabet, yielding 24 characters).
-   */
-  def randomBoundary: String = {
-    val array = new Array[Byte](18)
-    multipartBoundaryRandom.nextBytes(array)
-    Base64.custom.encodeToString(array, false)
-  }
-
   implicit def multipartMarshaller[T <: Multipart](implicit log: LoggingAdapter = NoLogging): ToEntityMarshaller[T] =
     Marshaller strict { value ⇒
-      val boundary = randomBoundary
+      val boundary = randomBoundary()
       val contentType = ContentType(value.mediaType withBoundary boundary)
       Marshalling.WithOpenCharset(contentType.mediaType, { charset ⇒
-        value match {
-          case x: Multipart.Strict ⇒
-            val data = BodyPartRenderer.strict(x.strictParts, boundary, charset.nioCharset, partHeadersSizeHint = 128, log)
-            HttpEntity(contentType, data)
-          case _ ⇒
-            val chunks = value.parts
-              .transform(() ⇒ BodyPartRenderer.streamed(boundary, charset.nioCharset, partHeadersSizeHint = 128, log))
-              .flatten(FlattenStrategy.concat)
-            HttpEntity.Chunked(contentType, chunks)
-        }
+        value.toEntity(charset, boundary)(log)
       })
     }
+
+  /**
+   * The random instance that is used to create multipart boundaries. This can be overriden (e.g. in tests) to
+   * choose how a boundary is created.
+   */
+  protected def multipartBoundaryRandom: java.util.Random = ThreadLocalRandom.current()
+
+  /**
+   * The length of randomly generated multipart boundaries (before base64 encoding). Can be overridden
+   * to configure.
+   */
+  protected def multipartBoundaryLength: Int = 18
+
+  /**
+   * The method used to create boundaries in `multipartMarshaller`. Can be overridden to create custom boundaries.
+   */
+  protected def randomBoundary(): String =
+    BodyPartRenderer.randomBoundary(length = multipartBoundaryLength, random = multipartBoundaryRandom)
 }
 
 object MultipartMarshallers extends MultipartMarshallers