The 26-year-old second-row forward becomes coach Peter Sharp's fourth signing for 2007, joining Matt Sing, Hutch Maiva and Wayne Godwin on the books at the KC Stadium.
The Tongan international made 22 appearances for the Tigers this year, scoring eight tries, although his season was punctuated after he was ordered to serve community service in Australia following an incident in 2005.
Manu will not affect Hull's quota for overseas players due to his Tongan passport.
"Willie is a big unit who has good feet,'' said Sharp. “He will give us plenty of attacking options that's for sure and we're looking forward to welcoming him to the squad.
"He is an excellent runner out wide and is a forward who will certainly contribute to our pack. As with all the other signings we have announced, we are very pleased to have him on board."