Millwall’s stalemate with Stoke earlier in the day moved them four points clear of the drop zone, leaving the Millers needing a win to stay alive.
They were putting the pressure on Neil Harris’ side when Clark Robertson headed them ahead early in the second half.
But their resistance was broken by goals in quick succession from Jay Rodriguez and Rekeem Harper late on to send the Millers back down to League One.
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Rotherham’s preparations were hampered by an illness to Jerry Yates, meaning boss Paul Warne could only name six substitutes.
They started like a side that knew they needed a result, compared to a West Brom team who already have a play-off spot secured.
Former Baggies defender Billy Jones fired an early shot wide and then Semi Ajayi headed Anthony Forde’s cross over at the near post.
The Millers almost took the lead in the 17th minute as Robertson headed Joe Newell’s corner against the woodwork.
Rotherham were on top and Robertson then got to another set-piece first, this time from Forde, only to glance it over.
Craig Dawson then denied the Millers the all-important breakthrough on the stroke of half-time.
The defender was in the right place at the right time to head off the line after Kyle Bartley almost diverted Will Vaulks’ long throw into his own net.
Rotherham, now playing with the wind, got the goal their play deserved just five minutes into the second half.
Robertson, a constant threat from set-pieces, headed Newell’s delivery back across goal and the Baggies defenders could not keep it out.
The goal sparked a sedate West Brom into action and the recalled Rodriguez hooked an effort over.
His striker partner Dwight Gayle then scuffed a shot wide on the turn after Ahmed Hegazi found him in the box.
Matty Phillips was introduced for the final 23 minutes and the midfielder soon set up Mason Holgate, who fired over at the back post under the challenge of Joe Mattock.
Marek Rodak then came to the rescue, tipping Holgate’s effort over as the Baggies pressed for an equaliser.
They got one from the penalty spot with 14 minutes remaining, with Rodriguez sending Rodak the wrong way after Gayle went down in the box.
Two minutes later and the Millers’ fate was sealed as Harper drove in his first goal for the Baggies from the edge of the area.
Phillips and Hal Robson-Kanu both failed to beat Rodak in one-in-ones on the break.
Substitute Kyle Vassell headed over and shot at Sam Johnstone while Newell struck the crossbar as the visitors tried in vain to respond.
Rotherham: Rodak, Mattock, Vaulks (Towell 71), Ajayi, Forde (Williams 83), Robertson, Crooks, Ihiekwe, Newell, Smith, Jones (Vassell 83).
Subs: Price, Wood, Taylor.
West Brom: Johnstone, Bartley, Dawson, Hegazi, Holgate (Murphy 78), Brunt, Harper, Johansen (Phillips 67), Gibbs, Rodriguez (Robson-Kanu 86), Gayle.
Subs: Bond, Adarabioyo, Townsend, Field, Murphy.
Referee: Robert Jones