diff --git a/llvm/docs/CMake.rst b/llvm/docs/CMake.rst
--- a/llvm/docs/CMake.rst
+++ b/llvm/docs/CMake.rst
@@ -625,6 +625,14 @@
   -DLLVM_EXTERNAL_FOO_SOURCE_DIR=/src/foo
   -DLLVM_EXTERNAL_BAR_SOURCE_DIR=/src/bar``.
 
+**LLVM_EXTERNAL_RUNTIMES**:STRING
+  Semicolon-separated list of additional external runtimes to build. For each
+  additional runtime LLVM_EXTERNAL_<NAME>_SOURCE_DIR has to be specified with
+  the path for the source code of the project. Example:
+  ``-DLLVM_EXTERNAL_RUNTIMES="mylib1;mylib2"
+  -DLLVM_EXTERNAL_MYLIB1_SOURCE_DIR=/src/mylib1
+  -DLLVM_EXTERNAL_MYLIB2_SOURCE_DIR=/src/mylib2``.
+
 **LLVM_EXTERNALIZE_DEBUGINFO**:BOOL
   Generate dSYM files and strip executables and libraries (Darwin Only).
   Defaults to OFF.
diff --git a/llvm/runtimes/CMakeLists.txt b/llvm/runtimes/CMakeLists.txt
--- a/llvm/runtimes/CMakeLists.txt
+++ b/llvm/runtimes/CMakeLists.txt
@@ -15,6 +15,19 @@
   STRING(REGEX REPLACE "-" "_" canon_name ${canon_name})
   set(LLVM_EXTERNAL_${canon_name}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../../${proj}")
 endforeach()
+if(LLVM_EXTERNAL_RUNTIMES)
+  string(REPLACE ";" "|" LLVM_EXTERNAL_RUNTIMES_PASSTHROUGH "${LLVM_EXTERNAL_RUNTIMES}")
+  list(APPEND EXTERNAL_RUNTIMES_CMAKE_ARGS -DLLVM_EXTERNAL_RUNTIMES=${LLVM_EXTERNAL_RUNTIMES_PASSTHROUGH})
+endif()
+foreach(proj ${LLVM_EXTERNAL_RUNTIMES})
+  string(TOUPPER "${proj}" canon_name)
+  STRING(REGEX REPLACE "-" "_" canon_name ${canon_name})
+  set(LLVM_EXTERNAL_${canon_name}_SOURCE_DIR
+    "${LLVM_EXTERNAL_${canon_name}_SOURCE_DIR}"
+    CACHE PATH "Path to ${proj} source directory")
+  list(APPEND EXTERNAL_RUNTIMES_CMAKE_ARGS
+    "-DLLVM_EXTERNAL_${canon_name}_SOURCE_DIR=${LLVM_EXTERNAL_${canon_name}_SOURCE_DIR}")
+endforeach()
 
 function(get_compiler_rt_path path)
   foreach(entry ${runtimes})
@@ -240,6 +253,7 @@
                                       -DCMAKE_CXX_COMPILER_WORKS=ON
                                       -DCMAKE_ASM_COMPILER_WORKS=ON
                                       ${COMMON_CMAKE_ARGS}
+                                      ${EXTERNAL_RUNTIMES_CMAKE_ARGS}
                                       ${RUNTIMES_CMAKE_ARGS}
                                       ${ARG_CMAKE_ARGS}
                            PASSTHROUGH_PREFIXES LLVM_ENABLE_RUNTIMES
@@ -348,6 +362,7 @@
                                       -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON
                                       -DLLVM_RUNTIMES_TARGET=${name}
                                       ${COMMON_CMAKE_ARGS}
+                                      ${EXTERNAL_RUNTIMES_CMAKE_ARGS}
                                       ${${name}_extra_args}
                            EXTRA_TARGETS ${${name}_extra_targets}
                                          ${${name}_test_targets}
diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -41,6 +41,17 @@
   set(LLVM_EXTERNAL_${canon_name}_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
 endforeach()
 
+foreach(proj ${LLVM_EXTERNAL_RUNTIMES})
+  string(TOUPPER "${proj}" canon_name)
+  STRING(REGEX REPLACE "-" "_" canon_name ${canon_name})
+  set(proj_dir "${LLVM_EXTERNAL_${canon_name}_SOURCE_DIR}")
+  if(IS_DIRECTORY ${proj_dir} AND EXISTS ${proj_dir}/CMakeLists.txt)
+    list(APPEND runtimes ${proj_dir})
+  else()
+    message(FATAL_ERROR "LLVM_EXTERNAL_RUNTIMES requests ${proj} but directory not found: ${proj_dir}")
+  endif()
+endforeach()
+
 function(runtime_register_component name)
   set_property(GLOBAL APPEND PROPERTY SUB_COMPONENTS ${name})
 endfunction()