Tony Mowbray’s side made an electric start as goals from Jack Clarke and Pierre Ekwah put them two goals ahead inside seven minutes.
Ekwah then added a third with a long-range strike on the stroke of half-time, before Bradley Dack netted a fourth shortly after the interval.
Finally, Chris Rigg put the icing on the cake with fifth goal in stoppage-time by heading home Jewison Bennette’s cross.
Here’s how Phil Smith rated the Sunderland players at the Stadium of Light:
1. Anthony Patterson - 6
Got away with one poor save in the second half that drifted just wide of his post. Otherwise not troubled too much and broadly speaking, poise in possession was good. 6 Photo: Frank Reid
2. Trai Hume - 8
That Samuel Edozie was withdrawn at half time was a big feather in his cap given how dangerous the winger is. Won all his duels, put in a good cross for the first goal. Coped with Ryan Fraser in the second, too. Very good. 8 Photo: Frank Reid
3. Dan Ballard - 7
Another good showing, though Southampton could have exploited some big set piece chances against a relatively small Sunderland side. Ballard made lots of good interceptions and challenges, showed his pace and battled through a second-half knock well. 7 Photo: Frank Reid
4. Luke O’Nien - 8
Another very impressive display both in and out of possession that further vindicated Tony Mowbray’s decision to bring him into the starting XI from the start of the season. Getting better by the game. 8 Photo: Frank Reid