diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -354,7 +354,12 @@
 
 option(LLVM_ENABLE_LIBPFM "Use libpfm for performance counters if available." ON)
 
-option(LLVM_ENABLE_THREADS "Use threads if available." ON)
+# On z/OS, threads cannot be used because TLS is not supported.
+if (CMAKE_SYSTEM_NAME MATCHES "OS390")
+  option(LLVM_ENABLE_THREADS "Use threads if available." OFF)
+else()
+  option(LLVM_ENABLE_THREADS "Use threads if available." ON)
+endif()
 
 set(LLVM_ENABLE_ZLIB "ON" CACHE STRING "Use zlib for compression/decompression if available. Can be ON, OFF, or FORCE_ON")
 
@@ -932,6 +937,13 @@
   string(APPEND CMAKE_SHARED_LINKER_FLAGS " -shared")
 endif()
 
+# Build with _XOPEN_SOURCE on z/OS.
+if (CMAKE_SYSTEM_NAME MATCHES "OS390")
+  add_definitions("-D_XOPEN_SOURCE=600")
+  add_definitions("-D_OPEN_SYS") # Needed for process information.
+  add_definitions("-D_OPEN_SYS_FILE_EXT") # Needed for EBCDIC I/O.
+endif()
+
 # Build with _FILE_OFFSET_BITS=64 on Solaris to match g++ >= 9.
 if (UNIX AND ${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
           add_definitions("-D_FILE_OFFSET_BITS=64")
@@ -942,7 +954,7 @@
 # check its symbols. This is wasteful (the check was done when foo.so
 # was created) and can fail since it is not the dynamic linker and
 # doesn't know how to handle search paths correctly.
-if (UNIX AND NOT APPLE AND NOT ${CMAKE_SYSTEM_NAME} MATCHES "SunOS|AIX")
+if (UNIX AND NOT APPLE AND NOT CMAKE_SYSTEM_NAME MATCHES "SunOS|AIX|OS390")
   set(CMAKE_EXE_LINKER_FLAGS
       "${CMAKE_EXE_LINKER_FLAGS} -Wl,-allow-shlib-undefined")
 endif()
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
@@ -221,7 +221,7 @@
 
     # Pass -O3 to the linker. This enabled different optimizations on different
     # linkers.
-    if(NOT (${CMAKE_SYSTEM_NAME} MATCHES "Darwin|SunOS|AIX" OR WIN32))
+    if(NOT (CMAKE_SYSTEM_NAME MATCHES "Darwin|SunOS|AIX|OS390" OR WIN32))
       set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                    LINK_FLAGS " -Wl,-O3")
     endif()
@@ -249,11 +249,12 @@
                        LINK_FLAGS " -Wl,-z,discard-unused=sections")
         endif()
       elseif(NOT WIN32 AND NOT LLVM_LINKER_IS_GOLD AND
-             NOT ${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD|AIX")
+             NOT CMAKE_SYSTEM_NAME MATCHES "OpenBSD|AIX|OS390")
         # Object files are compiled with -ffunction-data-sections.
         # Versions of bfd ld < 2.23.1 have a bug in --gc-sections that breaks
         # tools that use plugins. Always pass --gc-sections once we require
         # a newer linker.
+        # TODO Revisit this later on z/OS.
         set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                      LINK_FLAGS " -Wl,--gc-sections")
       endif()
diff --git a/llvm/cmake/modules/GetHostTriple.cmake b/llvm/cmake/modules/GetHostTriple.cmake
--- a/llvm/cmake/modules/GetHostTriple.cmake
+++ b/llvm/cmake/modules/GetHostTriple.cmake
@@ -14,7 +14,9 @@
     else()
       set( value "i686-pc-windows-gnu" )
     endif()
-  elseif( CMAKE_HOST_SYSTEM_NAME STREQUAL AIX )
+  elseif( CMAKE_SYSTEM_NAME MATCHES "OS390" )
+    set( value "s390x-ibm-zos" )
+  elseif( CMAKE_SYSTEM_NAME STREQUAL AIX )
     # We defer to dynamic detection of the host AIX version.
     if( CMAKE_SIZEOF_VOID_P EQUAL 8 )
       set( value "powerpc64-ibm-aix" )
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -144,6 +144,10 @@
   endif(FUCHSIA OR UNIX)
 endif(WIN32)
 
+if (CMAKE_SYSTEM_NAME MATCHES "OS390")
+  set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
+endif()
+
 set(EXEEXT ${CMAKE_EXECUTABLE_SUFFIX})
 set(LTDL_SHLIB_EXT ${CMAKE_SHARED_LIBRARY_SUFFIX})
 
@@ -207,7 +211,7 @@
 
 # Pass -Wl,-z,defs. This makes sure all symbols are defined. Otherwise a DSO
 # build might work on ELF but fail on MachO/COFF.
-if(NOT (${CMAKE_SYSTEM_NAME} MATCHES "Darwin|FreeBSD|OpenBSD|DragonFly|AIX|SunOS" OR
+if(NOT (CMAKE_SYSTEM_NAME MATCHES "Darwin|FreeBSD|OpenBSD|DragonFly|AIX|SunOS|OS390" OR
         WIN32 OR CYGWIN) AND
    NOT LLVM_USE_SANITIZER)
   set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-z,defs")