Chris Wilder's return to Sheffield United proved a painful one as his Middlesbrough side's play-off hopes were dented by a heavy defeat at Bramall Lane.
Sander Berge gave the Blades the lead from close range midway through the first half before Billy Sharp tapped in a second two minutes later as the hosts controlled the first period.
Jack Robinson seemed to have sealed the points with a cool finish after John Egan had headed a corner back into his path, but Folarin Balogun pulled one back moments later after a slip from Blades keeper Wes Foderingham.
But Morgan Gibbs-White sealed United's victory with a superb fourth goal as he flicked home Ben Osborn's cross.