diff --git a/xla/python/pytree.cc b/xla/python/pytree.cc
index a374c2df6bff9..d5799b8695cb7 100644
--- a/xla/python/pytree.cc
+++ b/xla/python/pytree.cc
@@ -96,6 +96,7 @@ void PyTreeRegistry::Register(
   registration->to_iterable = std::move(to_iterable);
   registration->from_iterable = std::move(from_iterable);
   registration->to_iterable_with_keys = std::move(to_iterable_with_keys);
+  nb::ft_lock_guard lock(mu_);
   auto it = registrations_.emplace(type, std::move(registration));
   if (!it.second) {
     throw std::invalid_argument(
@@ -112,6 +113,7 @@ void PyTreeRegistry::RegisterDataclass(nb::object type,
   registration->type = type;
   registration->data_fields = std::move(data_fields);
   registration->meta_fields = std::move(meta_fields);
+  nb::ft_lock_guard lock(mu_);
   auto it = registrations_.emplace(type, std::move(registration));
   if (!it.second) {
     throw std::invalid_argument(absl::StrFormat(
@@ -222,6 +224,7 @@ PyTreeKind PyTreeRegistry::KindOfObject(
 
 /*static*/ const PyTreeRegistry::Registration* PyTreeRegistry::Lookup(
     nb::handle type) const {
+  nb::ft_lock_guard lock(mu_);
   auto it = registrations_.find(type);
   return it == registrations_.end() ? nullptr : it->second.get();
 }
@@ -419,6 +422,7 @@ nb::object PyTreeRegistry::FlattenOneLevelImpl(nb::handle x,
                                              void* arg) {
   PyTreeRegistry* registry = nb::inst_ptr<PyTreeRegistry>(self);
   Py_VISIT(Py_TYPE(self));
+  nb::ft_lock_guard lock(registry->mu_);
   for (const auto& [key, value] : registry->registrations_) {
     Py_VISIT(key.ptr());
     int rval = value->tp_traverse(visit, arg);
@@ -431,6 +435,7 @@ nb::object PyTreeRegistry::FlattenOneLevelImpl(nb::handle x,
 
 /* static */ int PyTreeRegistry::tp_clear(PyObject* self) {
   PyTreeRegistry* registry = nb::inst_ptr<PyTreeRegistry>(self);
+  nb::ft_lock_guard lock(registry->mu_);
   registry->registrations_.clear();
   return 0;
 }
diff --git a/xla/python/pytree.h b/xla/python/pytree.h
index fc16fdd40136c..f526893d8dc81 100644
--- a/xla/python/pytree.h
+++ b/xla/python/pytree.h
@@ -143,9 +143,10 @@ class PyTreeRegistry {
       return a.ptr() == b.ptr();
     }
   };
+  mutable nanobind::ft_mutex mu_;
   absl::flat_hash_map<nanobind::object, std::unique_ptr<Registration>, TypeHash,
                       TypeEq>
-      registrations_;
+      registrations_;  // Guarded by mu_
   bool enable_namedtuple_;
 
   static int tp_traverse(PyObject* self, visitproc visit, void* arg);