diff --git a/internal/impl/message_reflect.go b/internal/impl/message_reflect.go
index 1b9b16a4..31c19b54 100644
--- a/internal/impl/message_reflect.go
+++ b/internal/impl/message_reflect.go
@@ -85,9 +85,7 @@ func (mi *MessageInfo) makeKnownFieldsFunc(si structInfo) {
 	mi.oneofs = map[protoreflect.Name]*oneofInfo{}
 	for i := 0; i < md.Oneofs().Len(); i++ {
 		od := md.Oneofs().Get(i)
-		if !od.IsSynthetic() {
-			mi.oneofs[od.Name()] = makeOneofInfo(od, si, mi.Exporter)
-		}
+		mi.oneofs[od.Name()] = makeOneofInfo(od, si, mi.Exporter)
 	}
 
 	mi.denseFields = make([]*fieldInfo, fds.Len()*2)
diff --git a/proto/oneof_which_test.go b/proto/oneof_which_test.go
index bdf10bdb..a53ae30d 100644
--- a/proto/oneof_which_test.go
+++ b/proto/oneof_which_test.go
@@ -7,6 +7,7 @@ package proto_test
 import (
 	"testing"
 
+	test3openpb "google.golang.org/protobuf/internal/testprotos/test3"
 	testhybridpb "google.golang.org/protobuf/internal/testprotos/testeditions/testeditions_hybrid"
 	testopaquepb "google.golang.org/protobuf/internal/testprotos/testeditions/testeditions_opaque"
 	"google.golang.org/protobuf/proto"
@@ -187,3 +188,24 @@ func TestOpaqueWhich(t *testing.T) {
 		}
 	}
 }
+
+func TestSyntheticOneof(t *testing.T) {
+	msg := test3openpb.TestAllTypes{}
+	md := msg.ProtoReflect().Descriptor()
+	ood := md.Oneofs().ByName("_optional_int32")
+	if ood == nil {
+		t.Fatal("failed to find oneof _optional_int32")
+	}
+	if !ood.IsSynthetic() {
+		t.Fatal("oneof _optional_int32 should be synthetic")
+	}
+	if msg.ProtoReflect().WhichOneof(ood) != nil {
+		t.Error("oneof _optional_int32 should not have a field set yet")
+	}
+	msg.OptionalInt32 = proto.Int32(123)
+	if msg.ProtoReflect().WhichOneof(ood) == nil {
+		t.Error("oneof _optional_int32 should have a field set")
+	}
+}
+
+// TODO(stapelberg): add test variants for the Hybrid API and Opaque API.
diff --git a/testing/prototest/message.go b/testing/prototest/message.go
index def37bff..86682f2e 100644
--- a/testing/prototest/message.go
+++ b/testing/prototest/message.go
@@ -583,13 +583,8 @@ func testOneof(t testing.TB, m protoreflect.Message, od protoreflect.OneofDescri
 				// Set fields explicitly.
 				m.Set(fda, newValue(m, fda, 1, nil))
 			}
-			if !od.IsSynthetic() {
-				// Synthetic oneofs are used to represent optional fields in
-				// proto3. While they show up in protoreflect, WhichOneof does
-				// not work on these (only on non-synthetic, explicit oneofs).
-				if got, want := m.WhichOneof(od), fda; got != want {
-					t.Errorf("after setting oneof field %q:\nWhichOneof(%q) = %v, want %v", fda.FullName(), fda.Name(), got, want)
-				}
+			if got, want := m.WhichOneof(od), fda; got != want {
+				t.Errorf("after setting oneof field %q:\nWhichOneof(%q) = %v, want %v", fda.FullName(), fda.Name(), got, want)
 			}
 			for j := 0; j < od.Fields().Len(); j++ {
 				fdb := od.Fields().Get(j)