Hull's fine away form continued at Watford as another smash-and-grab win gave them renewed hope of making the Championship play-offs.
Corry Evans prodded them into a third-minute lead, though the hosts went on to control much of the game.
But Danny Graham was twice denied by keeper Brad Guzan, and Will Buckley hit a post, before Robert Koren headed a late clincher for the visitors.
Martin Taylor nodded home a consolation goal deep in injury time.
Hull stretch their unbeaten run on the road to 15 matches after making the perfect start to reinvigorate their promotion push.
Liam Rosenior crossed for Evans to poke the ball past Scott Loach for his second goal in Hull colours.
Andreas Weimann recorded Watford's first effort as his shot crept past the post before the Austrian crashed a volley wide.
But it was Graham's involvement that brought them closest to a leveller, as the big striker turned at the edge of the area and let fly with a shot that seemed destined for the top corner - until Guzan brilliantly turned it over the bar.
Watford were in firm control, but Koren's side-footed shot from close range was hacked off the line in a reminder of City's potency on the counter.
John Eustace thought he had equalised at the other end, only for the veteran's header to hit team-mate Weimann and bounce away to safety.
Though Watford bossed posesssion - and chances - they found Hull's defence hard to breach.
Substitute Marvin Sordell put Graham behind the defence but Guzan handed them a reprieve with another vital save to beat the shot to safety.
Koren nodded home Tom Cairney's centre on 90 minutes after the pair had broken clear, before Taylor gave Watford slim hope in stoppage time as Hull moved to within five points of the play-offs.