diff --git a/akka-actor-typed-tests/src/test/scala/akka/actor/typed/internal/ActorSystemSpec.scala b/akka-actor-typed-tests/src/test/scala/akka/actor/typed/internal/ActorSystemSpec.scala
index 88616a4f3c..b6b434f4e2 100644
--- a/akka-actor-typed-tests/src/test/scala/akka/actor/typed/internal/ActorSystemSpec.scala
+++ b/akka-actor-typed-tests/src/test/scala/akka/actor/typed/internal/ActorSystemSpec.scala
@@ -9,18 +9,19 @@ import scala.concurrent.Future
 import scala.concurrent.Promise
 import scala.concurrent.duration._
 import scala.util.control.NonFatal
-
 import org.scalatest.BeforeAndAfterAll
 import org.scalatest.concurrent.Eventually
 import org.scalatest.concurrent.ScalaFutures
 import org.scalatest.matchers.should.Matchers
 import org.scalatest.wordspec.AnyWordSpec
-
 import akka.Done
+import akka.actor.dungeon.Dispatch
 import akka.actor.{ Address, CoordinatedShutdown, InvalidMessageException }
 import akka.actor.testkit.typed.scaladsl.LogCapturing
 import akka.actor.testkit.typed.scaladsl.TestInbox
+import akka.actor.testkit.typed.scaladsl.TestProbe
 import akka.actor.typed.scaladsl.Behaviors
+import com.typesafe.config.ConfigFactory
 
 class ActorSystemSpec
     extends AnyWordSpec
@@ -31,14 +32,15 @@ class ActorSystemSpec
     with LogCapturing {
 
   override implicit val patienceConfig: PatienceConfig = PatienceConfig(1.second)
-  def system[T](behavior: Behavior[T], name: String) = ActorSystem(behavior, name)
+  def system[T](behavior: Behavior[T], name: String, props: Props = Props.empty) =
+    ActorSystem(behavior, name, ConfigFactory.empty(), props)
   def suite = "adapter"
 
   case class Probe(message: String, replyTo: ActorRef[String])
 
-  def withSystem[T](name: String, behavior: Behavior[T], doTerminate: Boolean = true)(
+  def withSystem[T](name: String, behavior: Behavior[T], doTerminate: Boolean = true, props: Props = Props.empty)(
       block: ActorSystem[T] => Unit): Unit = {
-    val sys = system(behavior, s"$suite-$name")
+    val sys = system(behavior, s"$suite-$name", props)
     try {
       block(sys)
       if (doTerminate) {
@@ -165,5 +167,28 @@ class ActorSystemSpec
         sys.address shouldBe Address("akka", "adapter-address")
       }
     }
+
+    case class WhatsYourMailbox(replyTo: ActorRef[String])
+    "use a custom mailbox type for the user guardian" in {
+      withSystem(
+        "guardian-mailbox",
+        Behaviors.receive[WhatsYourMailbox] {
+          case (context, WhatsYourMailbox(replyTo)) =>
+            replyTo ! context
+              .asInstanceOf[ActorContextImpl[_]]
+              .classicActorContext
+              .asInstanceOf[Dispatch]
+              .mailbox
+              .messageQueue
+              .getClass
+              .getName
+            Behaviors.same
+        },
+        props = MailboxSelector.bounded(5)) { implicit sys =>
+        val probe = TestProbe[String]()
+        sys ! WhatsYourMailbox(probe.ref)
+        probe.expectMessage("akka.dispatch.BoundedMailbox$MessageQueue")
+      }
+    }
   }
 }
diff --git a/akka-actor/src/main/scala/akka/actor/ActorRefProvider.scala b/akka-actor/src/main/scala/akka/actor/ActorRefProvider.scala
index 600fde67d4..4ca4fcaade 100644
--- a/akka-actor/src/main/scala/akka/actor/ActorRefProvider.scala
+++ b/akka-actor/src/main/scala/akka/actor/ActorRefProvider.scala
@@ -562,7 +562,10 @@ private[akka] class LocalActorRefProvider private[akka] (
       system,
       system.guardianProps.getOrElse(Props(classOf[LocalActorRefProvider.Guardian], guardianStrategy)),
       dispatcher,
-      defaultMailbox,
+      system.guardianProps match {
+        case Some(props) => system.mailboxes.lookup(props.mailbox)
+        case None        => defaultMailbox
+      },
       rootGuardian,
       rootPath / "user")
     cell.initChild(ref)