Neil Harris made a winning return to Millwall as his side stunned Championship play-off contenders Southampton at St Mary's.
It was a third defeat in four for Russell Martin's home side - who were unbeaten in 25 before that - as they lost ground on the automatic promotion places.
Japhet Tanganga's brave header was cancelled out by Che Adams in the first half but Zian Flemming's penalty just before the break proved the difference.
Millwall had lost their last four, leading to Joe Edwards' departure in midweek, but incoming boss Harris - starting his second spell in charge - steered the Lions to their first win at Southampton since victory at The Dell in August 1989.