diff --git a/reflect/protodesc/desc_init.go b/reflect/protodesc/desc_init.go
index aff6fd49..e67a1541 100644
--- a/reflect/protodesc/desc_init.go
+++ b/reflect/protodesc/desc_init.go
@@ -114,6 +114,27 @@ func (r descsByName) initMessagesDeclarations(mds []*descriptorpb.DescriptorProt
 	return ms, nil
 }
 
+// canBePacked returns whether the field can use packed encoding:
+// https://protobuf.dev/programming-guides/encoding/#packed
+func canBePacked(fd *descriptorpb.FieldDescriptorProto) bool {
+	if fd.GetLabel() != descriptorpb.FieldDescriptorProto_LABEL_REPEATED {
+		return false // not a repeated field
+	}
+
+	switch protoreflect.Kind(fd.GetType()) {
+	case protoreflect.MessageKind, protoreflect.GroupKind:
+		return false // not a scalar type field
+
+	case protoreflect.StringKind, protoreflect.BytesKind:
+		// string and bytes can explicitly not be declared as packed,
+		// see https://protobuf.dev/programming-guides/encoding/#packed
+		return false
+
+	default:
+		return true
+	}
+}
+
 func (r descsByName) initFieldsFromDescriptorProto(fds []*descriptorpb.FieldDescriptorProto, parent protoreflect.Descriptor, sb *strs.Builder) (fs []filedesc.Field, err error) {
 	fs = make([]filedesc.Field, len(fds)) // allocate up-front to ensure stable pointers
 	for i, fd := range fds {
@@ -142,7 +163,7 @@ func (r descsByName) initFieldsFromDescriptorProto(fds []*descriptorpb.FieldDesc
 			f.L1.Presence = resolveFeatureHasFieldPresence(f.Base.L0.ParentFile, fd)
 			// We reuse the existing field because the old option `[packed =
 			// true]` is mutually exclusive with the editions feature.
-			if fd.GetLabel() == descriptorpb.FieldDescriptorProto_LABEL_REPEATED {
+			if canBePacked(fd) {
 				f.L1.HasPacked = true
 				f.L1.IsPacked = resolveFeatureRepeatedFieldEncodingPacked(f.Base.L0.ParentFile, fd)
 			}