Paul Warne’s fourth-placed side struggled again at play-off rivals Charlton Athletic, coached by former Leeds United star Lee Bowyer.
A disappointed Warne said: “The first goal was a shambles in all honesty. The defending for the second wasn’t much better and that’s something we need to improve on.
“I don’t feel like we’re in a crisis, but we need to improve quickly.”
The Millers fell behind after 28 minutes when Josh Emmanuel’s poor attempt at a clearing header fell straight to Polish striker Michal Zyro, who volleyed his third goal of the season past a stunned Marek Rodak.
Earlier, the Millers’ were denied by the woodwork in the opening minute, after Joe Newell set up Will Vaulks, whose thunderous strike cannoned back off the crossbar.
Soon after, Vaulks went close again as he drove narrowly over.
Shortly before the break, Ryan Williams lashed in a shot which Ben Amos saved superbly.
The Addicks began the second period on top, and Zyro almost doubled his and his team’s tally in the 52nd minute when he flashed a shot inches past the far post.
Two minutes later it was 2-0, as Joe Aribo rose to head home Jake Forster-Caskey’s free-kick.
Aribo made it 3-0 when he comprehensively beat Rodak.
Richard Wood fired in a rebound as a late consolation.
Charlton Athletic: Amos, Konsa, Bauer, Pearce, Dasilva, Aribo, Fosu-Henry (Kaikai, 18), Forster-Caskey, Reeves, Ajose (Mavididi, 67), Zyro. Unused substitutes: Marshall, Magennis, Phillips, Sarr, Dijksteel.
Rotherham United: Rodak, Emmanuel, Wood, Ajayi, Purrington (Lavery, 84), Forde (Taylor, 67), Palmer, Vaulks, Newell, Smith, R Williams.
Unused substitutes: Ball, Price, Ihiekwe, Cummings, Wiles.
Referee: Graham Horwood.