Westerman was named Young Player of the Year in 2008
Hull FC have signed Joe Westerman from Castleford on a four-year deal for an undisclosed six-figure sum.
Loose forward Westerman, 20, is considered to be one of the game's brightest prospects.
Confirmation of the move follows weeks of speculation over the player's future after he was cut from England's train-on squad for the Four Nations.
Westerman said: "One of the reasons I've signed is to really develop and improve my game."
Hull FC say the fee paid is one of the biggest in the club's history. The club's current record is £250,000 paid to St Helens for Alan Hunte in 1997.
Hull FC coach Richard Agar described Westerman as "one of if not the most talented and naturally gifted young forwards in the British game".
Westerman made his debut for Castleford aged just 17, and has since claimed Young Player of the Year Awards for both National League One and Super League.
He has scored 36 tries and 154 goals in just 86 league appearances, with some experts likening him to former rugby league greats Andy Farrell and Paul Sculthorpe.
Joe is one of if not the most talented and naturally gifted young forwards in the British game
Hull FC coach Richard Agar
Last season he scored eight tries and kicked 82 goals.
Agar told the club website: "Joe's already has a wealth of Super League experience, has been on the periphery of the international scene for a couple of seasons and at just 20 years old, we feel he has the ability to be a dominant player in the competition over the next decade.
"This is a signing not just for now, but for the long-term future of the club and the team's development.
"To secure a player of Joe's undoubted talent in the current market, while still on contract, represents a significant coup for the club."
Westerman said: "This is just massive for me. I thought it was time to move on and to have a team like Hull come in for me and for a club of this size to give me the opportunity to play for them is unbelievable.
"The opportunity to work under Rich [Agar] and play with players like Mark O'Meley and Craig Fitzgibbon is great for any player and I think this is a great move for me."
"I also want to thank Castleford and all the players, coaches, staff and supporters for everything they've done for me, particularly Terry Matterson who gave me a chance from being young."
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