The Whites struck first when Patrick Bamford latched onto Tyler Roberts' through ball to beat Saints goalkeeper Alex McCarthy. The lead was doubled on 78 minutes when Stuart Dallas finished from the edge of the box and Raphinha rounded off the scoring with a stunning free-kick.
The defeat extends Southampton's awful run of form as of late as the Saints have won just once in their last 13 top-flight outings, sitting in 14th, just eight points above the relegation zone.
VAR denies Saints
The visitors thought they had won a penalty through Nathan Tella only to see referee Andre Marriner consult his pitch-side monitor and overturn his prior decision, which turned out to be correct as replays showed Tella kicked Diego Llorente rather than the other way around.
Southampton thought they had taken the lead as Che Adams slotted home James Ward Prowse's free-kick, but Marriner had ruled the whistle hadn't blown. That proved to take all of the life out of the Saints, who never recovered.
Raphinha tears Southampton apart
The Brazilian was unplayable as he dominated Jan Bednarek from start to finish. Switching sides to the right after Helder Costa replaced Jack Harrison, Raphinha gave Southampton something else to think about. His direct style, selfless runs and brilliance on the ball combined with his creativity makes him perhaps Leeds' most difficult player to defend.
Leeds in good shape to stay up, Southampton being dragged into relegation dogfight
The Whites are on 35 points after the victory, 13 points clear of 18th-placed Fulham, all but guaranteeing their safety for the 2021/22 Premier League season. Saints on the other hand have lost seven of eight since defeating Liverpool on January 4th. With upcoming games against Everton, Manchester City as well as fellow relegation candidates Brighton & Hove Albion coming up, it's imperative that Ralph Hassenhuttl get his side's form turned around as soon as possible.