Manolo Gabbiadini scored twice in his second Southampton appearance as his side beat Sunderland who missed the chance to move out of the relegation zone.
The Saints took the lead in the 30th minute when Gabbiadini converted from Ryan Bertrand's cross.
The Italian scored again seconds before the break with a brilliant turn and finish inside the penalty area.
Jason Denayer's own goal and Shane Long's simple finish late on gave the visitors a fully-deserved, comprehensive win.
Sunderland managed just one shot on target in the game, a tame 30-yard strike from Didier Ndong.
The win for Claude Puel's side is just their second in the Premier League since Christmas and their first point on the road in 2017.