diff --git a/libcxx/modules/CMakeLists.txt b/libcxx/modules/CMakeLists.txt
index 0dea8cfca94ac..8dd1b51aac2f7 100644
--- a/libcxx/modules/CMakeLists.txt
+++ b/libcxx/modules/CMakeLists.txt
@@ -206,9 +206,15 @@ add_custom_target(generate-cxx-modules
 # Configure the modules manifest.
 # Use the relative path between the installation and the module in the json
 # file. This allows moving the entire installation to a different location.
+cmake_path(ABSOLUTE_PATH LIBCXX_INSTALL_LIBRARY_DIR
+  BASE_DIRECTORY ${CMAKE_INSTALL_PREFIX}
+  OUTPUT_VARIABLE ABS_LIBRARY_DIR)
+cmake_path(ABSOLUTE_PATH LIBCXX_INSTALL_MODULES_DIR
+  BASE_DIRECTORY ${CMAKE_INSTALL_PREFIX}
+  OUTPUT_VARIABLE ABS_MODULES_DIR)
 file(RELATIVE_PATH LIBCXX_MODULE_RELATIVE_PATH
-  ${CMAKE_INSTALL_PREFIX}/${LIBCXX_INSTALL_LIBRARY_DIR}
-  ${CMAKE_INSTALL_PREFIX}/${LIBCXX_INSTALL_MODULES_DIR})
+  ${ABS_LIBRARY_DIR}
+  ${ABS_MODULES_DIR})
 configure_file(
   "modules.json.in"
   "${LIBCXX_LIBRARY_DIR}/libc++.modules.json"