diff --git a/internal/encoding/tag/tag.go b/internal/encoding/tag/tag.go
index 80e8488b..948ca8b2 100644
--- a/internal/encoding/tag/tag.go
+++ b/internal/encoding/tag/tag.go
@@ -170,7 +170,9 @@ func Marshal(fd pref.FieldDescriptor, enumName string) string {
 	if fd.IsPacked() {
 		tag = append(tag, "packed")
 	}
-	// TODO: Weak fields?
+	if fd.IsWeak() {
+		tag = append(tag, "weak="+string(fd.Message().FullName()))
+	}
 	name := string(fd.Name())
 	if fd.Kind() == pref.GroupKind {
 		// The name of the FieldDescriptor for a group field is
diff --git a/internal/testprotos/test/test.pb.go b/internal/testprotos/test/test.pb.go
index bae3e177..f8c7e8cd 100644
--- a/internal/testprotos/test/test.pb.go
+++ b/internal/testprotos/test/test.pb.go
@@ -1469,7 +1469,7 @@ func (x *TestRequiredGroupFields) GetRepeatedgroup() []*TestRequiredGroupFields_
 }
 
 type TestWeak struct {
-	WeakMessage   *weak.WeakImportMessage `protobuf:"bytes,1,opt,name=weak_message,json=weakMessage" json:"weak_message,omitempty"`
+	WeakMessage   *weak.WeakImportMessage `protobuf:"bytes,1,opt,weak=goproto.proto.test.weak.WeakImportMessage,name=weak_message,json=weakMessage" json:"weak_message,omitempty"`
 	sizeCache     protoimpl.SizeCache
 	unknownFields protoimpl.UnknownFields
 }