New Bournemouth boss Kevin Bond was left to rue a second successive defeat in his two games in charge.
United's top scorer Will Hoskins set the scene for his side with a shot in the first minute, which was saved by Gareth Stewart in the home goal.
Eugen Bopp opened the scoring for Rotherham after 18 minutes. The midfielder picked up the ball in the centre of the field and ran forward unchallenged before striking a right-foot shot from 25 yards.
Bournemouth were forced into an early change when Marcus Browning limped off, but substitute Danny Hollands almost turned it to their advantage when he headed a Darren Anderton corner against the crossbar.
Parity lasted barely two minutes before Hoskins superbly glanced home a Michael Keane cross with his head into the bottom corner.
Rotherham extended their lead with a wonder-goal from Lee Williamson.
Hollands almost got one back for Bournemouth before half-time, but his point-blank header from a pass by Franck Songo'o was brilliantly saved by Neil Cutler in the Rotherham goal.
Bond made changes at half-time, bringing on Steven Foley-Sheridan for Neil Young and switching to a back three.
The new system suited Bournemouth well and they fought to get back into the game.
A Songo'o cross was headed over by Steve Fletcher from ten yards out, before the skipper saw a second effort tipped over by Cutler.
Foley-Sheridan should have pulled one back for Bournemouth when he met a cross from Songo'o with a free header from ten yards, but the midfielder could only nod wide.
He then shot wide after being sent clear by James Hayter, before Fletcher headed the final chance of the afternoon over the bar after more good work from Anderton, with the Millers going close again through Hoskins' shot which was beaten away by Stewart before the full time whistle sounded.