Bali Mumba’s goal lit up a poor first half as the Millers’ two-year stay in the Championship came to an end with five games still to play.
Argyle – who sacked head coach Ian Foster on Monday after eight losses in 11 games – eased their own relegation fears with the win.
It moves them up to 19th in the Championship, four points from safety, and means the Millers return to League One with an unassailable 19-point gap to 21st-place Birmingham City with only 15 points left to play for.
Interim Argyle boss Neil Dewsnip made four changes to his side, with all four dropping out being January signings by Foster as Dan Scarr and Callum Wright returned having barely featured under the former head coach.
In a first half that was low on quality, Argyle had the better of the chances – Wright twice went close after four Argyle players also threatened during a 10th-minute goalmouth scramble.
Mumba brought the game to life with a lovely touch to control Morgan Whittaker’s precise cross-field pass from the right before blasting past Viktor Johansson to give the Pilgrims a lead they just about deserved.
Argyle’s confidence began to grow after the goal and Wright forced a smart save from Johansson with a low far-post header in first-half stoppage time.
The Millers had to be grateful to their Swedish keeper as time and again he saved them – he tipped Whittaker’s powerful low effort around the post before stopping a Jordan Houghton shot.
Rotherham had their best chances midway through the second half as first Cohen Bramall hit the side netting having found himself unmarked at the back post from a corner before Sebastian Revan saw Michael Cooper produce his only save of the night soon after.
Johansson again came to the rescue moments later after Cameron Humphreys had got a toe on an Adam Randell cross.
He then saved a Dan Scarr header from a corner that saw Waine head against the post from the follow up before the New Zealand striker missed a gilt-edged chance when he capitalised on Sebastian Revan’s poor back-pass, but Johansson was off his line quickly to save with his foot as the Millers were relegated with a whimper.
