diff --git a/flask/helpers.py b/flask/helpers.py
index 8a65c2e9..6211b4aa 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
@@ -512,6 +512,8 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
 
     .. versionchanged:: 1.1
         Filenames may be a `PathLike` object.
+    .. versionadded:: 1.1
+        Partial content supports ``BytesIO``.
 
     :param filename_or_fp: the filename of the file to send.
                            This is relative to the :attr:`~Flask.root_path`
@@ -600,6 +602,12 @@ 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:
+                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..70b7349c 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 +609,23 @@ class TestSendfile(object):
         assert rv.status_code == 200
         rv.close()
 
+    @pytest.mark.skipif(
+        not callable(getattr(Range, 'to_content_range_header', None)),
+        reason="not implemented within werkzeug"
+    )
+    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"