diff --git a/synapse/rest/client/v1/login.py b/synapse/rest/client/v1/login.py
index 694072693d02..0d5eafd0fadc 100644
--- a/synapse/rest/client/v1/login.py
+++ b/synapse/rest/client/v1/login.py
@@ -74,16 +74,23 @@ def on_POST(self, request):
 
     @defer.inlineCallbacks
     def do_password_login(self, login_submission):
-        if not login_submission["user"].startswith('@'):
-            login_submission["user"] = UserID.create(
-                login_submission["user"], self.hs.hostname).to_string()
+        if 'medium' in login_submission and 'address' in login_submission:
+            user_id = yield self.hs.get_datastore().get_user_id_by_threepid(
+                login_submission['medium'], login_submission['address']
+            )
+        else:
+            user_id = login_submission['user']
+
+        if not user_id.startswith('@'):
+            user_id = UserID.create(
+                user_id, self.hs.hostname).to_string()
 
         token = yield self.handlers.auth_handler.login_with_password(
-            user_id=login_submission["user"],
+            user_id=user_id,
             password=login_submission["password"])
 
         result = {
-            "user_id": login_submission["user"],  # may have changed
+            "user_id": user_id,  # may have changed
             "access_token": token,
             "home_server": self.hs.hostname,
         }