diff --git a/llvm/docs/CommandGuide/lit.rst b/llvm/docs/CommandGuide/lit.rst
--- a/llvm/docs/CommandGuide/lit.rst
+++ b/llvm/docs/CommandGuide/lit.rst
@@ -523,6 +523,8 @@
  %S                      source dir (directory of the file currently being run)
  %p                      same as %S
  %{pathsep}              path separator
+ %{fsroot}               root component of file system paths
+ %{fssep}                file system path separator
  %t                      temporary file name unique to the test
  %basename_t             The last path component of %t but without the ``.tmp`` extension
  %T                      parent directory of %t (not unique, deprecated, do not use)
diff --git a/llvm/docs/TestingGuide.rst b/llvm/docs/TestingGuide.rst
--- a/llvm/docs/TestingGuide.rst
+++ b/llvm/docs/TestingGuide.rst
@@ -569,6 +569,13 @@
 
    Expands to the path separator, i.e. ``:`` (or ``;`` on Windows).
 
+``${fsroot}``
+   Expands to the root component of file system paths, i.e. ``/`` on Unix
+   systems or ``C:\`` on Windows.
+
+``${fssep}``
+   Exp``ands to the file system separator, i.e. ``/`` or ``\`` on Windows.
+
 ``%/s, %/S, %/t, %/T:``
 
   Act like the corresponding substitution above but replace any ``\``
diff --git a/llvm/utils/lit/lit/TestRunner.py b/llvm/utils/lit/lit/TestRunner.py
--- a/llvm/utils/lit/lit/TestRunner.py
+++ b/llvm/utils/lit/lit/TestRunner.py
@@ -1119,6 +1119,14 @@
                           ('%basename_t', baseName),
                           ('%T', tmpDir)])
 
+    fs_root = os.path.sep
+    if kIsWindows:
+        fs_root = 'C:\\'
+    substitutions.extend([
+        ('%{fsroot}', fs_root),
+        ('%{fssep}', os.path.sep),
+    ])
+
     # "%/[STpst]" should be normalized.
     substitutions.extend([
             ('%/s', sourcepath.replace('\\', '/')),