Middlesbrough's winless start to the Championship campaign continued as last season's play-off semi-finalists were beaten at West Bromwich Albion.
The Baggies took the lead midway through the first half when Cedric Kipre chested down and powered in a half-volley and John Swift doubled the lead six minutes later with powerful shot into the top corner after driving forward from midfield.
Emmanuel Latte Lath headed in Sammy Silvera's cross to immediately pull one back with his first goal for Boro, but Brandon Thomas-Asante restored West Brom's two-goal lead early in the second half after latching on to a backpass and rounding goalkeeper Seny Dieng before sliding in.
The visitors had Tommy Smith sent off in the 51st minute for a second bookable offence but had hope of a point when Marcus Forss made it 3-2 from the penalty spot with five minutes left after Darnell Furlong had been penalised for handball.
However, substitute Jeremy Sarmiento secured all three points for Carlos Corberan's men when the Ecuador international curled into the far corner in the first minute of added time.
It is now nine games without a win for Michael Carrick's Middlesbrough, including their two-legged tie against Coventry City in the play-offs in May.
The Teessiders look a shadow of the side which finished fourth last season, with Chuba Akpom - the second tier's top scorer in 2022-23 with 28 goals - sold to Dutch giants Ajax earlier this month.
West Brom, meanwhile, extended their unbeaten run to three Championship games following their opening-day defeat at Blackburn and have now won 12 of their 17 league games at The Hawthorns since Corberan was appointed as boss last October.
Thomas-Asante netted for the third successive game when he capitalised on an error by Lukas Engels and then Smith, who was booked for a foul on Jed Wallace in the first half, soon received his marching orders for a tug-back on the lively Matt Phillips.
Isaiah Jones looked bright after coming on for Boro, hitting a post and seeing his cross hit the arm of Furlong to allow Forss to pull one back from 12 yards, but Alex Mowatt teed up Sarmiento to seal victory.