27,000 watch Millers plunge to worst loss of season: Derby County 6-1 Rotherham United
It was their heaviest defeat of the season, the biggest of Paul Warne's tenure and it leaves them two points adrift of safety with seven games left.
Martyn Waghorn plundered a hat-trick, two of them penalties, while Bradley Johnson, Mason Mount and Duane Holmes also got on the scoresheet.
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Hide AdRichard Wood's second-half strike was nothing but a consolation for the Millers, who will have to quickly bounce back.
The writing was on the wall as early as the 13th minute when Waghorn converted his first spot-kick of the day, coming after Mount took a tumble under Ben Wiles' challenge – where contact looked minimal.
Two goals in three minutes before half-time killed the game as Johnson headed home Scott Malone's cross and then Waghorn finished off a quick counter-attacking move.
Mount made it four three minutes after the restart with a tidy finish from close range, before Wood headed home from a corner.
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Hide AdThere was never any hope of a comeback, though, as Holmes tucked home just after the hour and Waghorn completed his treble when he converted his second penalty, coming after Tom Lawrence was tripped by goalkeeper Marek Rodak.
That was the end of the Millers' pain, but this was a costly afternoon in their bid to retain their second-tier status.
Derby: Roos; Bogle, Keogh, Tomori, Malone (Cole 76), Holmes, Johnson, Mount (Bennett 76), Wilson, Waghorn (Marriott 81), Lawrence
Unused subs: Carson, Wisdom, Bryson, Huddlestone
Rotherham: Rodak, Vyner, Ihiekwe, Wood, Jones, Ajayi, Taylor (Towell 46), Williams, Wiles (Crooks 46), Newell, Yates (Forde 78)
Unused subs: Price, Palmer, Smith, Kayode
Referee: Tim Robinson
Attendance: 27,003