The Millers won only their 4th home game of the season by hammering the Posh 4-0,
The match featured two own goals and both teams finished with 10 men, but Rotherham were dominant throughout to cement their promotion credentials.
Adam Thompson should have sent Rotherham in ahead at the break but his back-post header was off target.
It took until the 50th minute for the home side's pressure to finally pay off when Richard Wood nodded in from Carlton Morris's cross.
Things got worse for Peterborough in the 54th minute when Frankie Kent received a second yellow for hauling down Chiedozie Ogbene.
Rotherham scored a second in the 68th minute, with Matt Crooks's effort deflected past Christy Pym by defender Daniel Butler.
Joe Mattock received a straight red for Rotherham on 73 minutes for forcing his head into the face of Ivan Toney.
A 77th-minute a scramble saw Joe Ward turn the ball into his own net for Rotherham's third before Kyle Vassell's acrobatic strike in the second minute of added time finished off the rout.