The Baggies started brightly as they chased a victory that would have put them right back in the survival picture.
West Brom And Boro Bite The Dust - One Place Left!
But a Shelton Martis error let Steven Gerrard in, and the new Football Writers' Player of the Year calmly chipped the ball over Dean Kiely to put the visitors ahead.
West Brom continued to press after the interval, with Marc-Antoine Fortune going close several times and having a goal rightly chalked off for offside, but Dirk Kuyt added a second to seal the home side's fate.
Gareth Southgate admitted he had been deeply hurt after being booed off the pitch as Middlesbrough's slim survival hopes suffered a mortal blow at the Riverside.
The Middlesbrough manager was targeted by a large section of fans after the 1-1 draw with Aston Villa.
Relegation is not a mathematical certainty but Boro are down.
Asked if the booing affected his desire to stay at the club whatever, Southgate said: "It has an impact I have to say.
"But I think emotions run high after a game like that, mine were high and so were the emotions of the supporters.
"Whatever happens at the end of the season, I am going to go on holiday and if we didn't stay in this league I would want to take the team back up, I'm pretty clear on that.
"But sometimes a reaction like that has a short-term impact on you - there's no question.
"I live with this hour by hour, not a day goes by when I don't wake up thinking about it, and not an evening goes by without me going to bed think about it.
"I would be lying if I said it didn't hurt, but I am big enough to deal with it."