diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index cfcf1404d82b7..569c01b61daac 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -150,6 +150,13 @@ if ("flang" IN_LIST LLVM_ENABLE_PROJECTS)
   endif()
 endif()
 
+if ("libc" IN_LIST LLVM_ENABLE_PROJECTS)
+  message(WARNING "Using LLVM_ENABLE_PROJECTS=libc is deprecated now, and will "
+    "become a fatal error in the LLVM 21 release.  Please use "
+    "-DLLVM_ENABLE_RUNTIMES=libc or see the instructions at "
+    "https://libc.llvm.org/ for building the runtimes.")
+endif()
+
 # Select the runtimes to build
 #
 # As we migrate runtimes to using the bootstrapping build, the set of default runtimes