Match ends, West Ham United 2, Southampton 1.
Andy Carroll came on to score the winner as West Ham recorded their first Premier League victory in two months by fighting back to beat Southampton.
Saints dominated the first half and went ahead when Dusan Tadic's prodded effort deflected in off Carl Jenkinson.
Michail Antonio levelled in bizarre fashion when a clearance hit him and looped in, before Carroll struck.
The West Ham striker made amends for a terrible miss by heading in from close range after Antonio had hit the bar.
Victory was the Hammers' first since a 2-1 win at home to Chelsea on 24 October, when then Blues boss Jose Mourinho was sent to the stands.
Slaven Bilic's side move up to seventh in the table, while Ronald Koeman's Saints, who beat Arsenal 4-0 at home on Boxing Day, stay 12th.
Re-live West Ham's 2-1 win over Southampton
Follow reaction to Monday's matches
Hammers finally have attacking threat
After drawing for a fifth time in a row on Saturday, Hammers boss Bilic bemoaned the injuries to his key attacking players.
Carroll and Manuel Lanzini made their returns as second-half substitutes on Monday, but the Croat is still without Victor Moses, Dimitri Payet, and Diafra Sakho up front.
Having scored just four times in their previous eight games, the comeback looked far from likely, but West Ham transformed the match through a second-half performance that was a world apart from their poor opening to the game.
There was a huge element of fortune about Antonio's equaliser, but it was deserved for the number of chances the home side created after the break, with Carroll missing the most clear-cut when earlier blazing over from six yards out.
Not quite the turning point for Southampton
The way Southampton started the match made you think they had finally turned the corner and rediscovered the kind of form that saw them finish seventh last year.
Koeman's side had picked up only one point from five league games before Saturday's demolition of Arsenal at St Mary's and that result had clearly given them confidence.
In the first half Saints had zip and zeal in midfield, were solid at the back, and created chances aplenty through the running of Shane Long and Sadio Mane up front.
But if the away side were far superior in the opening 45 minutes, the opposite was true in the second half as the introduction of Carroll and Lanzini turned the match in the Hammers' favour.
Man of the match - Michail Antonio
What the managers said:
West Ham boss Slaven Bilic: "After the goal we were totally down and I waited for half-time because I didn't want to humiliate the players by changing them before. I was happy when the referee blew, because they could have been 2-0 up.
"At half-time we made some changes but it was a change of character, of approach, of determination and will. They did try in the first half, but this was a different West Ham.
"We equalised and continued to chase, and at the end of the day we were rewarded for everything we did in the second half."
Southampton manager Ronald Koeman: "The second half was totally different to the first. We had to score more than one in the first half, because otherwise you expect their reaction in the second. If you score only one they still have belief.
"The first half was the level of the Arsenal game. It was one team on the pitch playing football. But out of a better second half they scored two goals, and from a fantastic first we scored one.
"If you are the dominant side you have to score more than one goal, and that was the problem."
West Ham play Liverpool at home in a 12:45 GMT kick-off on Saturday, when Southampton travel to Norwich for a 15:00 start.
- 5TomkinsBooked at 58mins
- 4SongSubstituted forLanziniat 45'minutes
- 11E ValenciaSubstituted forObiangat 86'minutes
- 10ZárateSubstituted forCarrollat 45'minutes
- 15MartinaBooked at 52minsSubstituted forYoshidaat 70'minutes
- 6José Fonte
- 17van Dijk
- 21BertrandBooked at 66mins
- 14RomeuBooked at 56mins
- 11TadicSubstituted forRamírezat 75'minutes
- 10ManéSubstituted forJuanmiat 75'minutes
- Michael Oliver
- Shots on Target
Second Half ends, West Ham United 2, Southampton 1.
Attempt missed. Virgil van Dijk (Southampton) right footed shot from the centre of the box misses to the left.
Attempt missed. Michail Antonio (West Ham United) right footed shot from the centre of the box is high and wide to the right. Assisted by Carl Jenkinson following a fast break.
Corner, Southampton. Conceded by James Tomkins.
Substitution, West Ham United. Pedro Obiang replaces Enner Valencia because of an injury.
Offside, Southampton. Ryan Bertrand tries a through ball, but Juanmi is caught offside.
Delay over. They are ready to continue.
Delay in match Enner Valencia (West Ham United) because of an injury.
Corner, Southampton. Conceded by Angelo Ogbonna.
James Tomkins (West Ham United) wins a free kick in the defensive half.
Foul by Shane Long (Southampton).
Goal! West Ham United 2, Southampton 1. Andy Carroll (West Ham United) header from very close range to the top left corner.
Michail Antonio (West Ham United) hits the bar with a right footed shot from the centre of the box.
Offside, Southampton. Jose Fonte tries a through ball, but Shane Long is caught offside.
Attempt missed. Steven Davis (Southampton) right footed shot from outside the box misses to the right following a corner.
Corner, Southampton. Conceded by James Tomkins.
Attempt missed. James Tomkins (West Ham United) right footed shot from outside the box misses to the left following a set piece situation.
Enner Valencia (West Ham United) wins a free kick in the attacking half.
Foul by Virgil van Dijk (Southampton).