diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -471,6 +471,7 @@
 # 'config-ix' use them during feature checks. It also adds them to both
 # 'LIBCXX_COMPILE_FLAGS' and 'LIBCXX_LINK_FLAGS'
 if(ZOS)
+  # Make sure EBCDIC encoding is used by default on z/OS.
   add_target_flags_if_supported("-fzos-le-char-mode=ebcdic")
 endif()
 
diff --git a/libcxx/cmake/caches/zos.cmake b/libcxx/cmake/caches/zos.cmake
new file mode 100644
--- /dev/null
+++ b/libcxx/cmake/caches/zos.cmake
@@ -0,0 +1,15 @@
+set(CMAKE_BUILD_TYPE Release CACHE STRING "")
+
+set(LIBCXX_ENABLE_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_SHARED ON CACHE BOOL "")
+set(LIBCXX_ENABLE_STATIC OFF CACHE BOOL "")
+set(LIBCXX_INCLUDE_TESTS ON CACHE BOOL "")
+set(LIBCXX_ASCII_ZOS_BUILD ON CACHE BOOL
+    "Build ascii version of libcxx library on z/OS.")
+set(LIBCXXABI_ENABLE_SHARED ON CACHE BOOL "")
+set(LIBCXXABI_ENABLE_STATIC OFF CACHE BOOL "")
+set(LIBCXXABI_INCLUDE_TESTS ON CACHE BOOL "")
+
+set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
+set(LLVM_ENABLE_WERROR ON CACHE BOOL "")
diff --git a/libcxx/include/__config_site.in b/libcxx/include/__config_site.in
--- a/libcxx/include/__config_site.in
+++ b/libcxx/include/__config_site.in
@@ -10,7 +10,18 @@
 #define _LIBCPP___CONFIG_SITE
 
 #cmakedefine _LIBCPP_ABI_VERSION @_LIBCPP_ABI_VERSION@
+#if defined(__MVS__)
+#include <features.h> // for __NATIVE_ASCII_F
+#define _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_X##_LIBCPP_Y
+#define _LIBCPP_CONCAT(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y)
+#ifdef __NATIVE_ASCII_F
+#cmakedefine _LIBCPP_ABI_NAMESPACE _LIBCPP_CONCAT(@_LIBCPP_ABI_NAMESPACE@,_a)
+#else
+#cmakedefine _LIBCPP_ABI_NAMESPACE _LIBCPP_CONCAT(@_LIBCPP_ABI_NAMESPACE@,_e)
+#endif // __NATIVE_ASCII_F
+#else
 #cmakedefine _LIBCPP_ABI_NAMESPACE @_LIBCPP_ABI_NAMESPACE@
+#endif
 #cmakedefine _LIBCPP_ABI_FORCE_ITANIUM
 #cmakedefine _LIBCPP_ABI_FORCE_MICROSOFT
 #cmakedefine _LIBCPP_HAS_NO_THREADS
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -218,6 +218,26 @@
   cxx_add_common_build_flags(cxx_shared)
   cxx_set_common_defines(cxx_shared)
 
+  if(ZOS)
+    if(${CMAKE_CXX_COMPILER_TARGET} STREQUAL "s390x-ibm-zos")
+      # 64 bit
+      set(dll_name   "CRTEQCXE")
+      set(dll_name_a "CRTEQCXS")
+    else()
+      # 32 bit
+      set(dll_name   "CRTEHCXE")
+      set(dll_name_a "CRTEHCXS")
+    endif()
+
+    add_custom_command(TARGET cxx_shared POST_BUILD
+      COMMAND
+        ${LIBCXX_SOURCE_DIR}/utils/zos_rename_dll_side_deck.sh
+        $<TARGET_LINKER_FILE_NAME:cxx_shared> $<TARGET_FILE_NAME:cxx_shared> "${dll_name}"
+      COMMENT "Rename dll name inside the side deck file"
+      WORKING_DIRECTORY $<TARGET_FILE_DIR:cxx_shared>
+    )
+  endif()
+
   # Link against LLVM libunwind
   # Note that we do need to link against libunwind directly to ensure that the correct
   # dependencies are recorded when creating a linker script.
@@ -274,6 +294,40 @@
   endif()
 endif()
 
+# Build the ascii shared library for z/OS.
+if (LIBCXX_ASCII_ZOS_BUILD)
+
+  add_library(cxx_ascii SHARED ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
+  target_include_directories(cxx_ascii PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
+  target_link_libraries(cxx_ascii PUBLIC cxx-headers
+                                  PRIVATE ${LIBCXX_LIBRARIES})
+  set_target_properties(cxx_ascii
+    PROPERTIES
+      COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS} -fzos-le-char-mode=ascii"
+      LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
+      OUTPUT_NAME   "c++_a"
+      VERSION       "${LIBCXX_ABI_VERSION}.0"
+      SOVERSION     "${LIBCXX_ABI_VERSION}"
+      DEFINE_SYMBOL ""
+      POSITION_INDEPENDENT_CODE ON
+  )
+  cxx_add_common_build_flags(cxx_ascii)
+  cxx_set_common_defines(cxx_ascii)
+
+  add_custom_command(TARGET cxx_ascii POST_BUILD
+    COMMAND
+      ${LIBCXX_SOURCE_DIR}/utils/zos_rename_dll_side_deck.sh
+      $<TARGET_LINKER_FILE_NAME:cxx_ascii> $<TARGET_FILE_NAME:cxx_ascii> "${dll_name_a}"
+    COMMENT "Rename dll name inside the side deck file"
+    WORKING_DIRECTORY $<TARGET_FILE_DIR:cxx_ascii>
+  )
+
+  # Link against libc++abi
+  target_link_libraries(cxx_ascii PUBLIC "${LIBCXX_CXX_SHARED_ABI_LIBRARY}")
+
+  list(APPEND LIBCXX_BUILD_TARGETS "cxx_ascii")
+endif()
+
 set(CMAKE_STATIC_LIBRARY_PREFIX "lib")
 
 # Build the static library.
diff --git a/libcxx/utils/zos_rename_dll_side_deck.sh b/libcxx/utils/zos_rename_dll_side_deck.sh
new file mode 100755
--- /dev/null
+++ b/libcxx/utils/zos_rename_dll_side_deck.sh
@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+#
+# Script to rename DLL name within side deck.
+#
+
+# Stops execution if a command or pipeline has an error.
+set -e
+
+sidedeck=$1
+old_dll_name=$2
+new_dll_name=$3
+
+function error() {
+  printf "ERROR: %s\n" "$*"
+  exit 1
+}
+
+function usage() {
+cat <<EOF
+Usage: $(basename $0) <side deck file> <old dll name> <new dll name>:
+          [-h|--help] Display this help and exit.
+EOF
+}
+
+rename_dll_name_inside_side_deck() {
+
+if [[ -z "$sidedeck" || -z "$old_dll_name" || -z "$new_dll_name" ]]; then
+  usage
+  error "All 3 parameters must be specified."
+fi
+
+[[ -f "$sidedeck" ]] || error "The '$sidedeck' file must exists."
+
+old_len=${#old_dll_name}
+new_len=${#new_dll_name}
+
+if (( $new_len > $old_len )); then
+  error "New DLL name $new_dll_name must have $old_len characters or less."
+fi
+
+if ((padding_len=$old_len-$new_len )); then
+  pad=$(printf "%*s" $padding_len "")
+fi
+
+# Touch the temp. file and set the tag to 1047 first so the redirecting statement
+# will write in 1047 and not 819 encoding.
+touch $sidedeck.tmp; chtag -tc1047 $sidedeck.tmp
+sed "/ IMPORT /s/'$old_dll_name/$pad'$new_dll_name/g" $sidedeck > $sidedeck.tmp
+mv $sidedeck.tmp $sidedeck
+}
+
+function main() {
+  rename_dll_name_inside_side_deck
+}
+
+main "$@"
+
diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -238,6 +238,7 @@
 
 # Configure target flags
 if(ZOS)
+  # Make sure EBCDIC encoding is used by default on z/OS.
   add_target_flags_if_supported("-fzos-le-char-mode=ebcdic")
 endif()