On-the-whistle report: West Brom 2 Rotherham United 1
The Millers were leading the Baggies in the second half at the Hawthorns but two quickfire goals for the home side sealed their fate.
Paul Warne's men had been in the bottom three since mid-February yet had hung on in the scrap for survival.
Clark Robertson's 50th-minute header gave them hope of extending their fight until the final week of the season but it wasn't to be and their stay in the second tier is over after one season following goals from Jay Rodriguez and Rekeem Harper.
Rotherham were the more dangerous side in the opening stages and twice could have taken the lead.
Semi Ajayi headed over Anthony Forde's 10th-minute cross and Clark Roberton was much closer to scoring seven minutes later.
The centre-half met Joe Newell's inswinging corner but his header smacked the top of the post.
The Millers almost went ahead on the stroke of half-time when Will Vaulks launched a long throw into the box and Craig Dawson had to clear Kyle Bartley's inadvertent back-header off the line.
They needed only five minutes of the second half to take the lead when Robertson rose to head home Joe Newell's free-kick.
A West Brom defender tried desperately to hook the ball away but the referee was given the message from his linesman that it was already over the line and the goal stood.
The Millers were grateful that Rodgriguez and Dwight Gayle fluffed their lines at the other end, both making a mess of their connections and failing to hit the target.
Mason Holgate had no problem connecting with Matt Phillips' 69th-minute cross but blazed over and Marek Rodak was called into action for the first time on 73 minutes when he had to tip over Holgate's cross-cum-lob.
All he could four minutes later, however, was pick the ball out of the net after Rodriguez had despatched a penalty awarded for a foul on Gayle.
And worse was to come within a couple of minutes as Rakeem Harper found the bottom corner to put West Brom 2-1 ahead.
Kyle Vassell almost pulled back a goal with a header late on and Newell hit the bar but the Baggies missed one-on-one chances through Matt Phillips and Hal Kanu-Robson.
It was a grimly apt way to end a season during which the Millers have now given up 32 points from winning positions.
West Brom (3-5-2): Sam Johnstone; Craig Dawson, Kyle Bartley, Ahmed Hegazi; Mason Holgate, Rekeem Harper, Stefan Johansen, Chris Brunt, Kieran Gibbs; Dwight Gayle, Jay Rodriguez. Unused: Jonathan Bond, Conor Townsend, Tosin Adarabioyo, Sam Field.
Subs: Matt Phillips (for Johansen, 67), Jacob Murphy (for Holgate, 77), Hal Robson-Kanu (for Rodriguez, 86).
Rotherham (4-1-4-1): Marek Rodak; Billy Jones, Michael Ihiekwe, Clark Robertson, Joe Mattock; Semi Ajayi; Anthony Forde, Will Vaulks, Matt Crooks, Joe Newell; Michael Smith. Unused: Lewis Price, Richard Wood, Jon Taylor.
Subs: Richie Towell (for Vaulks, 71), Ryan Williams (for Forde, 83), Kyle Vassell (for Jones, 83).
Goals: Rodriguez pen 77, Harper 79 (West Brom); Robertson 50 (Rotherham).
Referee: Robert Jones (Merseyside).