diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -1659,7 +1659,10 @@
         set(${base_dir} ${LIT_BASE_DIR} PARENT_SCOPE)
         return()
       else()
-        message(WARNING "LLVM_EXTERNAL_LIT set to ${LLVM_EXTERNAL_LIT}, but the path does not exist.")
+        if (NOT DEFINED CACHE{LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE})
+          message(WARNING "LLVM_EXTERNAL_LIT set to ${LLVM_EXTERNAL_LIT}, but the path does not exist.")
+          set(LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE YES CACHE BOOL "")
+        endif()
       endif()
     endif()
   endif()