New Rotherham boss Leam Richardson tasted defeat in his first match in charge, as second-half goals from Grady Diangana and Jed Wallace gave West Brom a comfortable win.
The ex-Wigan manager was appointed on Monday, replacing Matt Taylor, who was sacked in November.
After a quiet first half, Diangana scored the opener when the ball was cleared to him on the edge of the box and he fired a deflected shot past keeper Viktor Johansson in the 54th minute.
Sam Nombe flashed a header wide in the 84th minute as Rotherham looked for an equaliser, before Jed Wallace's excellent late free-kick wrapped up the points for the visitors.