Swansea City 0 Southampton 1
Northern Ireland trio Jonny Evans, Chris Brunt and Gareth McAuley have suffered relegation with West Brom after Southampton's 1-0 victory over Swansea City last night.
The Saints took a huge step towards safety while edging their Welsh rivals towards the Premier League trapdoor.
West Brom's drop to the Championship was confirmed, ending their eight-year stay in the top flight.
It also deals a huge blow to the Northern Ireland internationals, however skipper Evans is expected to move to a Premier League side this summer.
There is a clause in Evans' contract that means he can leave for £3m if West Brom are relegated and there will now be a scramble for his signature.
Both managers made positive substitutions over the course of the contest as Tammy Abraham was sent on for Swansea and Shane Long and Manolo Gabbiadini provided Southampton with extra attacking options.
Abraham brought a save from Alex McCarthy, but it was Gabbiadini's goalmouth instinct that proved crucial as he was the first to react when Lukasz Fabianski pushed out Charlie Austin's shot.
The hosts huffed and puffed for an opening after that, but their seven-season stay in the Premier League is now in real jeopardy.
"It was a push to us because the boys did everything in the game," said Swansea boss Carlos Carvalhal.
"They played with a fantastic attitude. We played more long balls than normal but it is normal in this kind of situation with the emotion. The goalkeeper made two or three fantastic saves.
"I will not put the fault in my players, I never do this. I assume responsibility. It is not a question they are not good enough, we created chances. We did our maximum, the players did their maximum but we did not score for whatever reason. In this moment we are not relegated. Tomorrow, we don't know. Definitely we don't depend on ourselves.
"We are professional, it is not easy this situation. We must react very fast because we have a game on Sunday."
Saints striker Austin said: "We knew we had to come here and win. I think I had eight or nine chances but it didn't matter who scored.
"It's massive. I have been relegated before with QPR and it's not a nice feeling. The gaffer has come in and got some belief into us."
Gabbiadini had only been on the pitch for four minutes and it was only his fifth goal of a difficult season for the Italian.
West Brom needed a draw here to stay alive until the final game, but their brave bid under caretaker manager Darren Moore is over and the Baggies will spend next season in the Sky Bet Championship.
Southampton had looked dead and buried when Mark Hughes came in to replace the sacked Mauricio Pellegrino in March.
But seven points from three games has given Saints a huge opportunity to escape the drop on the final day this weekend, even if their game is a home date against Manchester City.
Southampton now have 36 points - three more than Swansea, who entertain relegated Stoke on Sunday.
The Saints' goal difference is also nine better than Swansea's, so they will be safe barring a significant turnaround.
Huddersfield also remain in the mix given the poor state of their goal difference, although one point from remaining games against Chelsea and Arsenal will ensure they finish above Swansea.
Swansea: Fabianski, Naughton (Narsingh, 75 mins), Fernandez, Mawson, Roberts, Ki, King (Carroll, mins), Clucas, Olsson (Abraham, mins), A Ayew, J Ayew. Unused subs: van der Hoorn, Dyer, Nordfeldt, Sanches.
Southampton: McCarthy, Bednarek (Gabbiadini, 68 mins), Stephens, Hoedt, Cedric, Hojbjerg, Romeu, Bertrand, Tadic (McQueen, 83 mins), Redmond (Long, 64 mins), Austin. Unused subs: Carrillo, Ward-Prowse, Sims, Forster.
Referee: Michael Oliver (Northumberland)
Man of the match: Charlie Austin
Match rating: 6/10