diff --git a/llvm/runtimes/CMakeLists.txt b/llvm/runtimes/CMakeLists.txt
index c44795536af5..ed96a593329a 100644
--- a/llvm/runtimes/CMakeLists.txt
+++ b/llvm/runtimes/CMakeLists.txt
@@ -717,6 +717,76 @@ if(build_runtimes)
           EXTRA_ARGS TARGET_TRIPLE ${name} ${extra_args})
       endforeach()
     endforeach()
+
+    # Generate a default multilib.yaml for each runtime target that has one.
+    if(LLVM_RUNTIME_MULTILIBS)
+      set(_multilib_yaml_outputs)
+      foreach(name ${LLVM_RUNTIME_TARGETS})
+        set(_multilibs_for_target)
+        foreach(multilib ${LLVM_RUNTIME_MULTILIBS})
+          if("${name}" IN_LIST LLVM_RUNTIME_MULTILIB_${multilib}_TARGETS)
+            list(APPEND _multilibs_for_target "${multilib}")
+          endif()
+        endforeach()
+
+        # Generate the file into a temporary. We only copy this if the user has
+        # not provided one either in the build tree or the install directory.
+        if(_multilibs_for_target)
+          set(_yaml [=[
+MultilibVersion: 1.0
+
+Variants:
+- Dir: .
+  Flags: []
+]=])
+          set(_flag_values "")
+          foreach(multilib ${_multilibs_for_target})
+            string(APPEND _yaml "\
+- Dir: ${multilib}
+  Flags: [-fmultilib-flag=${multilib}]
+")
+            string(APPEND _flag_values "\
+  - Name: ${multilib}
+")
+          endforeach()
+          string(APPEND _yaml "
+Mappings: []
+
+Flags:
+- Name: multilib
+  Values:
+  - Name: none
+${_flag_values}\
+  Default: none
+")
+
+          set(_staged_dir "${CMAKE_CURRENT_BINARY_DIR}/multilib-yaml/${name}")
+          set(_staged "${_staged_dir}/multilib.yaml")
+          set(_dest "${LLVM_LIBRARY_DIR}/${name}/multilib.yaml")
+          file(GENERATE OUTPUT "${_staged}" CONTENT "${_yaml}")
+
+          # Omitting DEPENDS so the command only runs when OUTPUT is absent.
+          add_custom_command(
+            OUTPUT "${_dest}"
+            COMMAND "${CMAKE_COMMAND}" -E copy "${_staged}" "${_dest}"
+            COMMENT "Generating default multilib.yaml for ${name}")
+          list(APPEND _multilib_yaml_outputs "${_dest}")
+
+          install(CODE "
+            set(_inst \"\$ENV{DESTDIR}\${CMAKE_INSTALL_PREFIX}/lib/${name}/multilib.yaml\")
+            if(NOT EXISTS \"\${_inst}\")
+              file(INSTALL \"${_staged}\"
+                   DESTINATION \"\$ENV{DESTDIR}\${CMAKE_INSTALL_PREFIX}/lib/${name}\")
+            endif()
+          ")
+        endif()
+      endforeach()
+
+      if(_multilib_yaml_outputs)
+        add_custom_target(runtimes-multilib-yaml ALL DEPENDS ${_multilib_yaml_outputs})
+        set_target_properties(runtimes-multilib-yaml PROPERTIES FOLDER "Runtimes")
+      endif()
+    endif()
   endif()
 
   if(NOT LLVM_BUILD_INSTRUMENTED AND CLANG_ENABLE_BOOTSTRAP)