Carlton Cole's brace helped West Ham defeat Hull City at Upton Park, but it was not enough for the Hammers to secure automatic promotion.
Sam Allardyce's side were beaten to second spot by Southampton, who hammered Coventry 4-0, and will now face Cardiff City in the play-offs.
The first leg will be played in Wales on Thursday, with the second leg at Upton Park on Monday, 7 May.
Corry Evans scored a late consolation for Hull after a defensive mix-up.
Southampton's victory meant the match turned into an anti-climax, with the only positive for the Hammers being the momentum they will take into the play-offs from their display against the Tigers.
There was only ever going to be one winner once Cole headed home a corner in the 36th minute, and Hull only came back into the game once the Hammers started to conserve energy.
Cole doubled the lead when he peeled away from his marker and fired home from Henri Lansbury's pass four minutes after half-time - and the England striker was substituted moments later as Allardyce began to prepare for Thursday's semi-final first leg.
The Hammers dominated possession, looking to have banished memories of their seven-game winless stretch at home, but a mistake at the back allowed Hull back into the game.
A poor header from defender Guy Demel let in midfielder Evans, who nodded the ball home for his third goal of the season.
Richard Garcia thought he had won a point for Nick Barmby's men, but his late diving header was disallowed for offside.
However, anything but three points would have been harsh on the Hammers, who could have won by a greater margin if Lansbury and Kevin Nolan had showed better finishing earlier in the game when their shots failed to trouble Hull goalkeeper Vito Mannone.