Goals from Simon Cox and Roman Bednar condemned the Teessiders to their first defeat in nine games, and ensured Gordon Strachan's side will definitely be playing in the Championship next season.
Boro are now seven points behind sixth-placed Swansea City with just two games to play and therefore cannot make the play-offs.
They needed to win at the Hawthorns to have any chance, but that outcome never looked likely in a one-sided game that emphasised just how much summer rebuilding is needed on the banks of the Tees.
With promotion having been assured last weekend, West Brom's supporters were clearly in the mood to party, and barely eight minutes had elapsed when the first Mexican Wave of the afternoon rippled around the Hawthorns.
The mood in the away end was more fraught, as afternoon victories for Swansea and Blackpool meant Middlesbrough needed at least a point to have any chance of making the play-offs.
The visitors were at least boosted by the presence of Gary O'Neil, who ended a two-game absence after Boro officials agreed a pay-as-you-play agreement with Portsmouth officials late on Friday night.
Boro had originally been due to pay £1m if O'Neil made his 100th appearance for the club, but Portsmouth's administrator has agreed to accept a series of staggered payments for every game the midfielder plays between now and the end of the season.
O'Neil's return added some solidity to the Middlesbrough ranks, and the visitors successfully restricted their opponents to a handful of long-range strikes in the opening 25 minutes.
Chris Brunt produced the best of them, but Boro goalkeeper Brad Jones successfully parried the winger's 20-yard drive after Bednar had held the ball up well on the right-hand side.
Jones found himself clutching at thin air when Bednar shot again on the half-hour mark, but this time the woodwork came to his rescue.
West Brom's Czech striker turned inside both Stephen McManus and Kyle Naughton in the penalty area, but his rising drive crashed against the underside of the crossbar and rebounded to safety.
Boro's only effort on goal at that stage had been an early Rhys Williams strike that sailed miles over the crossbar, but Scott McDonald came close four minutes before the break with a prodded effort that went narrowly wide following a smart lay-off from his strike partner, Chris Killen.
Cox showed the Australian how to do it in first-half stoppage-time as the Baggies gained the advantage their first-half dominance warranted.
Bednar chested the ball into the striker's path, and after turning past McManus, he lashed a rising 18-yard drive into the top right-hand corner of the net.
The pair combined again to double the Baggies' advantage ten minutes after the interval, but this time it was Cox providing the assist and Bednar weighing in with the finish.
Robert Koren was given two opportunities to cross form the right, and after Cox headed the ball down, Bednar prodded past Jones from the edge of the six-yard box.
West Brom were the dominant side throughout the second half, and their win would have been even more emphatic had Jones not produced good saves to deny both Koren and Bednar in the closing stages of the game.
However, Boro came close themselves in stoppage time, with McDonald heading Barry Robson's free-kick against the crossbar.
WEST BROM (4-4-2): Carson (Kiely 46); Reid, Tamas, Olsson, Cech; Koren, Dorrans, Mulumbu, Brunt; Cox (Morrison 75), Bednar (Miller 82).
Subs (not used): Mattock, Meite, Slory, Barnes.
MIDDLESBROUGH (4-4-2): Jones; Hoyte, McManus, Grounds (Riggott 70), Naughton; Robson, R Williams (Franks 80), O'Neil, Arca; Killen (Lita 54), McDonald.
Subs (not used): Coyne (gk), Taylor, O'Shea, Miller.