diff --git a/flask/helpers.py b/flask/helpers.py
index e59f7d3c..a4785ed4 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -609,6 +609,9 @@ def send_from_directory(directory, filename, **options):
     :param options: optional keyword arguments that are directly
                     forwarded to :func:`send_file`.
     """
+    # Expand relative directories to the app's root_path
+    if not os.path.isabs(directory):
+        directory = os.path.join(current_app.root_path, directory)
     filename = safe_join(directory, filename)
     if not os.path.isfile(filename):
         raise NotFound()