diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 8b4cd2636fd4d..9f75f65d54af2 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -773,6 +773,8 @@ if (RUNTIMES_USE_LIBC STREQUAL "picolibc")
   config_define(1 _LIBCPP_LIBC_NEWLIB)
 elseif (RUNTIMES_USE_LIBC STREQUAL "newlib")
   config_define(1 _LIBCPP_LIBC_NEWLIB)
+elseif (RUNTIMES_USE_LIBC STREQUAL "llvm-libc")
+  config_define(1 _LIBCPP_LIBC_LLVM_LIBC)
 endif()
 
 # TODO: Remove in LLVM 21. We're leaving an error to make this fail explicitly.
diff --git a/libcxx/include/__config b/libcxx/include/__config
index b4db39472ce31..2cadce967d689 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -716,7 +716,7 @@ typedef __char32_t char32_t;
 #  endif
 
 #  if defined(__BIONIC__) || defined(__NuttX__) || defined(__Fuchsia__) || defined(__wasi__) ||                        \
-      _LIBCPP_HAS_MUSL_LIBC || defined(__OpenBSD__) || defined(__LLVM_LIBC__)
+      _LIBCPP_HAS_MUSL_LIBC || defined(__OpenBSD__) || _LIBCPP_LIBC_LLVM_LIBC
 #    define _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE
 #  endif
 
diff --git a/libcxx/include/__config_site.in b/libcxx/include/__config_site.in
index b09ca807ee812..a1cc1208aeff3 100644
--- a/libcxx/include/__config_site.in
+++ b/libcxx/include/__config_site.in
@@ -45,6 +45,7 @@
 // C libraries
 #cmakedefine01 _LIBCPP_LIBC_PICOLIBC
 #cmakedefine01 _LIBCPP_LIBC_NEWLIB
+#cmakedefine01 _LIBCPP_LIBC_LLVM_LIBC
 
 // __USE_MINGW_ANSI_STDIO gets redefined on MinGW
 #ifdef __clang__
diff --git a/libcxx/include/__random/binomial_distribution.h b/libcxx/include/__random/binomial_distribution.h
index 0712e4ef4a4f6..6d07ff3c7df04 100644
--- a/libcxx/include/__random/binomial_distribution.h
+++ b/libcxx/include/__random/binomial_distribution.h
@@ -98,7 +98,7 @@ class binomial_distribution {
 };
 
 // Some libc declares the math functions to be `noexcept`.
-#if _LIBCPP_GLIBC_PREREQ(2, 8) || defined(__LLVM_LIBC__)
+#if _LIBCPP_GLIBC_PREREQ(2, 8) || _LIBCPP_LIBC_LLVM_LIBC
 #  define _LIBCPP_LGAMMA_R_NOEXCEPT _NOEXCEPT
 #else
 #  define _LIBCPP_LGAMMA_R_NOEXCEPT
diff --git a/libcxx/src/chrono.cpp b/libcxx/src/chrono.cpp
index 20387ea76124b..a27efc5a42b5d 100644
--- a/libcxx/src/chrono.cpp
+++ b/libcxx/src/chrono.cpp
@@ -31,7 +31,7 @@
 #  include <sys/time.h> // for gettimeofday and timeval
 #endif
 
-#if defined(__LLVM_LIBC__)
+#if _LIBCPP_LIBC_LLVM_LIBC
 #  define _LIBCPP_HAS_TIMESPEC_GET
 #endif
 
diff --git a/libcxx/src/filesystem/filesystem_clock.cpp b/libcxx/src/filesystem/filesystem_clock.cpp
index 49f65efb5a53a..865a1018871f2 100644
--- a/libcxx/src/filesystem/filesystem_clock.cpp
+++ b/libcxx/src/filesystem/filesystem_clock.cpp
@@ -32,7 +32,7 @@
 #  include <sys/time.h> // for gettimeofday and timeval
 #endif
 
-#if defined(__LLVM_LIBC__)
+#if _LIBCPP_LIBC_LLVM_LIBC
 #  define _LIBCPP_HAS_TIMESPEC_GET
 #endif
 
diff --git a/libcxx/src/include/config_elast.h b/libcxx/src/include/config_elast.h
index be665a97bf91b..daec2429de8eb 100644
--- a/libcxx/src/include/config_elast.h
+++ b/libcxx/src/include/config_elast.h
@@ -21,7 +21,7 @@
 // where strerror/strerror_r can't handle out-of-range errno values.
 #if defined(ELAST)
 #  define _LIBCPP_ELAST ELAST
-#elif defined(__LLVM_LIBC__)
+#elif _LIBCPP_LIBC_LLVM_LIBC
 // No _LIBCPP_ELAST needed for LLVM libc
 #elif _LIBCPP_LIBC_NEWLIB
 #  define _LIBCPP_ELAST __ELASTERROR