diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -185,7 +185,15 @@
    For example, -DLIBCXX_LIBRARY_VERSION=x.y will result in the library being named
    libc++.x.y.dylib, along with the usual symlinks pointing to that. On Apple platforms,
    this also controls the linker's 'current_version' property.")
-set(LIBCXX_ABI_NAMESPACE "__${LIBCXX_ABI_VERSION}" CACHE STRING "The inline ABI namespace used by libc++. It defaults to __n where `n` is the current ABI version.")
+
+set(_default_abi_namespace "__${LIBCXX_ABI_VERSION}")
+if (LIBCXX_ENABLE_DEBUG_MODE)
+  set(_default_abi_namespace "${_default_abi_namespace}d")
+endif()
+set(LIBCXX_ABI_NAMESPACE "${_default_abi_namespace}" CACHE STRING
+  "The inline ABI namespace used by libc++. It defaults to __n where `n` is the current ABI version.
+   If the debug mode is enabled, the 'd' suffix is added to the namespace to distinguish it from the
+   non-debug version of the library.")
 if (NOT LIBCXX_ABI_NAMESPACE MATCHES "__.*")
   message(FATAL_ERROR "LIBCXX_ABI_NAMESPACE must be a reserved identifier.")
 endif()