diff --git a/CHANGES.rst b/CHANGES.rst
index 7d8369f7..3d198aec 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -10,15 +10,19 @@ Version 1.1
 Unreleased
 
 -   :meth:`flask.RequestContext.copy` includes the current session
-    object in the request context copy. This prevents ``flask.session``
-    pointing to an out-of-date object. (`#2935`)
--   Using built-in RequestContext, unprintable Unicode characters in Host
-    header will result in a HTTP 400 response and not HTTP 500 as previously.
-    (`#2994`)
--   :func:`send_file` supports ``PathLike`` objects as describe in
-    PEP 0519, to support ``pathlib`` in Python 3. (`#3059`_)
+    object in the request context copy. This prevents ``session``
+    pointing to an out-of-date object. (`#2935`_)
+-   Using built-in RequestContext, unprintable Unicode characters in
+    Host header will result in a HTTP 400 response and not HTTP 500 as
+    previously. (`#2994`_)
+-   :func:`send_file` supports :class:`~os.PathLike` objects as
+    described in PEP 0519, to support :mod:`pathlib` in Python 3.
+    (`#3059`_)
+-   :func:`send_file` supports :class:`~io.BytesIO` partial content.
+    (`#2957`_)
 
 .. _#2935: https://github.com/pallets/flask/issues/2935
+.. _#2957: https://github.com/pallets/flask/issues/2957
 .. _#2994: https://github.com/pallets/flask/pull/2994
 .. _#3059: https://github.com/pallets/flask/pull/3059
 
diff --git a/flask/helpers.py b/flask/helpers.py
index 8a65c2e9..5afba8d5 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -8,7 +8,7 @@
     :copyright: © 2010 by the Pallets team.
     :license: BSD, see LICENSE for more details.
 """
-
+import io
 import os
 import socket
 import sys
@@ -511,7 +511,10 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
         compatibility with WSGI servers.
 
     .. versionchanged:: 1.1
-        Filenames may be a `PathLike` object.
+        Filename may be a :class:`~os.PathLike` object.
+
+    .. versionadded:: 1.1
+        Partial content supports :class:`~io.BytesIO`.
 
     :param filename_or_fp: the filename of the file to send.
                            This is relative to the :attr:`~Flask.root_path`
@@ -600,6 +603,13 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
             mtime = os.path.getmtime(filename)
             fsize = os.path.getsize(filename)
             headers['Content-Length'] = fsize
+        elif isinstance(file, io.BytesIO):
+            try:
+                fsize = file.getbuffer().nbytes
+            except AttributeError:
+                # Python 2 doesn't have getbuffer
+                fsize = len(file.getvalue())
+            headers['Content-Length'] = fsize
         data = wrap_file(request.environ, file)
 
     rv = current_app.response_class(data, mimetype=mimetype, headers=headers,
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index e36e0e64..921a9942 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -10,6 +10,7 @@
 """
 
 import datetime
+import io
 import os
 import uuid
 
@@ -608,6 +609,19 @@ class TestSendfile(object):
         assert rv.status_code == 200
         rv.close()
 
+    def test_send_file_range_request_bytesio(self, app, client):
+        @app.route('/')
+        def index():
+            file = io.BytesIO(b'somethingsomething')
+            return flask.send_file(
+                file, attachment_filename='filename', conditional=True
+            )
+
+        rv = client.get('/', headers={'Range': 'bytes=4-15'})
+        assert rv.status_code == 206
+        assert rv.data == b'somethingsomething'[4:16]
+        rv.close()
+
     @pytest.mark.skipif(
         not callable(getattr(Range, 'to_content_range_header', None)),
         reason="not implemented within werkzeug"