diff --git a/src/_pytest/config/findpaths.py b/src/_pytest/config/findpaths.py
index 43c2367793e..807bcc2146b 100644
--- a/src/_pytest/config/findpaths.py
+++ b/src/_pytest/config/findpaths.py
@@ -203,7 +203,7 @@ def determine_setup(
                     else:
                         cwd = Path.cwd()
                     rootdir = get_common_ancestor([cwd, ancestor])
-                    is_fs_root = os.path.splitdrive(str(rootdir))[1] == "/"
+                    is_fs_root = os.path.splitdrive(str(rootdir))[1] == os.sep
                     if is_fs_root:
                         rootdir = ancestor
     if rootdir_cmd_arg: