Goals in the 62nd and 71st minutes from Lazar Markovic and Sam Clucas gave the Tigers a precious three points when a draw appeared the best they could have hoped for against a Watford side already safe from relegation.
Hull coach Marco Silva made two changes from the side which started at Stoke with Brazilian Evandro taking Tom Huddlestone's place in midfield and Ahmed Elmohamady coming in at right-back with central defender Michael Dawson rested to the bench.
Central defender Miguel Britos returned from suspension for Watford in place of Adrian Mariappa.
Hull settled after Harry Maguire conceded two early corners through misunderstandings with goalkeeper Eldin Jakupovic but chances were limited.
Then came the contentious dismissal, Niasse catching M'Baye Niang just below his right knee as they challenged for the bouncing ball from a Watford clearance. Contact appeared minimal but the Watford player went down and referee Robert Madley immediately issued a straight red card to the Senegal international.
Jakupovic kept the scoreline blank by palming the ball over the bar from Sebastian Prodl's back-header from a corner.
Britos headed wide from eight yards from Watford's fifth corner of the half and Alfred N'Diaye went in the book for grappling Nordin Amrabat as Hull kept it scoreless at the break.
Striker Abel Hernandez replaced Evandro for the start of the second half as Watford remained unchanged.
Kamil Grosicki's forceful run earned Hull their first corner before they struck with a stunning counter-attack.
Maguire headed out a free-kick, Hernandez knocked it clear when the ball was played back in and Andy Robertson sped down the right before feeding Gosicki, whose cross brought a flying header from Markovic as Gomes came out. The ball struck the bar and rebounded onto the line but Markovic rose and slammed the ball in from inside the six-yard area.
The Tigers hero then earned a 27-yard free-kick from which Clucas went close with a curling strike over the five-man wall.
It proved only a sighter as Clucas volleyed Hull 2-0 ahead after Watford had headed clear a corner to the midfield player 25 yards out - Gomes going full length to his left but getting nowhere near the ball in the 71st minute.
Huddlestone replaced Markovic in the 81st minute as the Tigers held out for an important win which keeps them above third-bottom Swansea City - Dawson also replacing Grosicki in the 89th minute.