Ipswich were knocked off the top of Sky Bet League One as Matt Crooks' double gave Rotherham a 2-0 away win.
The hosts were handed a second defeat in the space of four days as Crooks scored in each half to move the Millers to within a point of the play-off places.
Despite a strong opening, the Tractor Boys fell behind in the 11th minute when Crooks beat Tomas Holy with a fine strike from the edge of the box.
The Millers almost doubled their lead two minutes later when Joe Mattock's left-wing cross found Matthew Olosunde but the American's half-volley was too high.
Crooks scored his and Rotherham's second in the 48th minute, expertly slotting home from Ben Wiles' pass.
Visiting goalkeeper Daniel Iversen stuck out an arm to stop Luke Garbutt rounding him in the 71st minute before Chiedozie Ogbene went close to Rotherham's third after sliding in at the back post two minutes later.
The Millers continued to create chances and Michael Ihiekwe headed against a post from Wiles' corner with 12 minutes to go.