Mileta Rajovic scored twice as in-form Watford continued their fine record against Rotherham with a convincing win at Vicarage Road.
The Dane turned in two left-wing crosses by Ken Sema before the break while Edo Kayembe fired his first goal for the club after the turnaround and Tom Ince and Matheus Martins made it 5-0 late-on.
The Hornets are up to 13th and unbeaten in six, their best league run since April 2021, and have conceded just three goals in that spell.
The third-bottom Millers' winless away run is now 21 league games spanning more than a year, while they have taken just a point from their eight road games this season.