Haaland, absent from City’s previous league win over Liverpool with a groin injury, displayed his athleticism to meet Jack Grealish’s cross for City’s third and take his remarkable league tally this season to 30 goals in 27 games.
It was also his 44th goal in all competitions for City this season – equalling the most by a Premier League player in a single campaign.
Kevin de Bruyne registered his 100th Premier League assist as he set up Haaland to give dominant City a half-time lead at St Mary’s, before Grealish continued his impressive post-World Cup form as he beat Gavin Bazunu at the second time of asking.
Sekou Mara emerged from the Saints bench to reduce the deficit following Haaland’s acrobatic second, but just 75 seconds later City were awarded a penalty for a foul on De Bruyne which Julian Alvarez calmly converted.
It completed an eighth consecutive win in all competitions for Pep Guardiola’s side, who maintained the pressure on Arsenal before the Gunners face Liverpool at Anfield on Sunday (16:30 BST).
Southampton remain four points adrift of safety at the bottom of the table and time is running out for Ruben Selles and his players to rescue their situation.