diff --git a/llvm/lib/Support/Threading.cpp b/llvm/lib/Support/Threading.cpp
index 923935bbca103..7cc7ba44cc72d 100644
--- a/llvm/lib/Support/Threading.cpp
+++ b/llvm/lib/Support/Threading.cpp
@@ -83,6 +83,11 @@ unsigned llvm::ThreadPoolStrategy::compute_thread_count() const {
   // the same interface as std::thread but requests the same stack size as the
   // main thread (8MB) before creation.
 const std::optional<unsigned> llvm::thread::DefaultStackSize = 8 * 1024 * 1024;
+#elif defined(_AIX)
+  // On AIX, the default pthread stack size limit is ~192k for 64-bit programs.
+  // This limit is easily reached when doing link-time thinLTO. AIX library
+  // developers have used 4MB, so we'll do the same.
+const std::optional<unsigned> llvm::thread::DefaultStackSize = 4 * 1024 * 1024;
 #else
 const std::optional<unsigned> llvm::thread::DefaultStackSize;
 #endif