The 22-year-old former Huddersfield and Wakefield winger has been released from his contract by Castleford to enable him to join the Championship club.
Griffin, who has scored 14 tries in 21 appearances for the Tigers this season, will move to Headingley, where his older brother Darrell plays for Leeds Rhinos, on a two-year deal with immediate effect.
"It was a big decision for me to move away from Super League, having broken through in recent years, but I am excited about what Leeds Carnegie are doing," said Griffin, who is from Oxford and grew up playing rugby union before switching to league.
"It will take me a while to get up to speed with the game but I played union growing up down south so I am sure it will come back pretty quickly."
Meanwhile, Castleford, who finish the season at Hull on Saturday week, will have a host of players making their last home appearances against Catalan Dragons on Sunday.
Danny Orr, Ryan Hudson and Nick Youngquest, who are all retiring, and Stuart Jones will be making their last appearances at the Probiz Coliseum and the match could also mark the penultimate appearances of prop Paul Jackson and stand-off Rangi Chase, who remains a target for Hull.
"It will be a particularly emotional day on Sunday with seven players not contracted to the club for 2013 and some playing the last home game of their careers," said coach Ian Millward.
"We hope to give them all the send-off they deserve."
Chase and hooker Daryl Clarke are set to return from injury and half-back Jamie Ellis will be back from suspension as Millward's men look to end a run of five consecutive defeats and avoid the wooden spoon.
Chase, the reigning Man of Steel, has not played since picking up a knee injury in the home defeat by Warrington in July while Clarke has spent three months on the sidelines with a knee injury.
Millward said: "Rangi Chase has been training very strongly so he will definitely return to the starting line-up on Sunday.
"Daryl has also worked extremely hard to get back to fitness ahead of time which is testament to his attitude, work ethic and our physio Ben Sterling.
"There is no doubt the return of our three best playmakers will give us a much-needed boost for the Catalans game.
"Rangi and Jamie have only played together once this season so it will be great to see that combination back on the field."
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