diff --git third_party/sentencepiece/src/src/util.cc third_party/sentencepiece/src/src/util.cc
index c5e5289807a0c..e10880abc17eb 100644
--- third_party/sentencepiece/src/src/util.cc
+++ third_party/sentencepiece/src/src/util.cc
@@ -16,6 +16,7 @@
 
 #include <atomic>
 #include <iostream>
+#include <memory>
 
 namespace sentencepiece {
 
@@ -197,8 +198,18 @@ std::mt19937 *GetRandomGenerator() {
 }
 #else
 std::mt19937 *GetRandomGenerator() {
-  thread_local static std::mt19937 mt(GetRandomGeneratorSeed());
-  return &mt;
+  // Thread-locals occupy stack space in every thread ever created by the
+  // program, even if that thread never uses the thread-local variable.
+  //
+  // https://maskray.me/blog/2021-02-14-all-about-thread-local-storage
+  //
+  // sizeof(std::mt19937) is several kilobytes, so it is safer to put that on
+  // the heap, leaving only a pointer to it in thread-local storage.  This must
+  // be a unique_ptr, not a raw pointer, so that the generator is not leaked on
+  // thread exit.
+  thread_local static auto mt =
+      std::make_unique<std::mt19937>(GetRandomGeneratorSeed());
+  return mt.get();
 }
 #endif
 }  // namespace random