The full back, making his 133rd appearance for the Tigers, headed in the rebound after Nikica Jelavic’s penalty had been saved by Ben Foster.
Shane Long then added a second before the break to ensure City claimed a ninth victory of the season.
With Steve Bruce believing a double figure tally of wins will be enough to secure safety, Hull are looking good with an eight point cushion on the bottom three with eight games of the campaign remaining.
Not that Hull had it all their own way against the Baggies. Far from it, in fact, with West Brom matching the hosts in open play.
However, when it came to getting the breaks, the Tigers had the luck to leave Pepe Mel’s side looking nervously over their shoulders at the bottom three.
The major talking point of the afternoon came on the half-hour when the home side was awarded a penalty.
Chris Foy, such a controversial figure a week ago as Chelsea lost at Aston Villa, adjudged Craig Dawson to have tripped Shane Long after the City striker had nipped in front of his one-time team-mate.
There looked to be minimal contact and, to Long’s credit, he got immediately to his feet and tried to play on.
Foy, however, pointed to the spot after consulting his linesman. Nikica Jelavic took the penalty but Foster saved and Rosenior headed in the rebound.
At that stage, the game had largely been even. Afterwards, though, Hull took control as Foster saved brilliantly from Tom Huddlestone before Long doubled the home side’s advantage.
Jake Livermore started the move with a hook over his shoulder into the box that Curtis Davies nodded into the path of Long, who made no mistake from six yards against the club that sold him to City in January.
Hull had the first chance of the second half, too, as Aluko crossed for Jelavic tovolley into the side-netting.
Despite that, Albion actually started the better of the two and it took a Rosenior header off his own line to keep out Victor Anichebe.
Ahmed Elmohamady then had to be alert to avert the danger as James Morrison’s cross caused panic in the City back-line.
At the other end, Long hit the inside of the post before Rosenior again came to Hull’s rescue with Anichebe waiting to pounce.