The Irish midfielder becomes Peter Taylor's first signing of the summer following promotion from the third division.
Keane spent two months on loan with Town back in the 2002/03 season, but couldn't do enough to keep the Mariners in the first division.
During his seven game spell, Keane scored twice against Reading and Brighton. His passion saw him emerge as a fans favourite despite his short stay, whilst his celebration in front of the Brighton fans on the final day say him become Public Enemy Number One in Hove.
The 21-year-old seemed to have kickstarted his career at Deepdale last season but fell out of favour towards the end of the season and jumped at the chance to join the Tigers.
Speaking to the official Hull website Keane said: "I think it's a massive club, once I'd heard Hull was interested I never had any doubts about coming here.
Keane joins the Tigers
For starters I think it's the manager, he's a big, big manager and he's got great ambition.
"The stadium, which I've seen, and the supporters - its got huge support. It's a club going in the right direction."
"Obviously it's dropping down a division but I look at it as a stepping stone, it's like a ladder. I think this is a club that's definitely on the up.
"All you have to do is come here and have a look at the place and you understand what comparison's this club is at."