diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -48,9 +48,13 @@
   list(APPEND CMAKE_MODULE_PATH ${LLVM_DIR_ABSOLUTE})
 
   if(FLANG_BUILD_NEW_DRIVER)
+    # If the user specifies a relative path to CLANG_DIR, the calls to include
+    # Clang modules fail. Append the absolute path to CLANG_DIR instead.
+    get_filename_component(CLANG_DIR_ABSOLUTE ${CLANG_DIR} REALPATH)
+    list(APPEND CMAKE_MODULE_PATH ${CLANG_DIR_ABSOLUTEDIR})
+
     # TODO: Remove when libclangDriver is lifted out of Clang
-    list(APPEND CMAKE_MODULE_PATH ${CLANG_DIR})
-    find_package(Clang REQUIRED HINTS "${CLANG_DIR}")
+    find_package(Clang REQUIRED HINTS "${CLANG_DIR_ABSOLUTE}")
   endif()
 
   # If LLVM links to zlib we need the imported targets so we can too.