Rotherham United moved within two points of safety as they consigned Middlesbrough to a third straight home defeat.
The Millers started the stronger and Matt Crooks atoned for a big miss by slotting a low shot into the bottom corner from 20 yards.
Sam Morsy was given a gilt-edged chance to level, teed up by Duncan Watmore's thrilling run, but side-footed straight at goalkeeper Jamal Blackman.
Michael Smith converted a late penalty after keeper Marcus Bettinelli felled Freddie Ladapo, before a composed finish from debutant Ryan Giles capped their biggest win of the season.
Boro had the chance to move back into the play-off spots, but the Millers were ruthless on the counter-attack as they sealed a first win on Teesside since the 1964-65 season.