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The 25-year-old had been on a season-long loan from the Rhinos but will now join Cas on a four-and-a-half-year contract on 1 July.
He has scored four tries in 17 Super League games for the league leaders.
"It says a bit about us that we have been able to attract him. He's a big signing for us," coach Daryl Powell told BBC Radio Leeds.
"It took him a while in pre-season to pick up how we play and he has just been getting better and better.
"You want to sign high-quality players for as long as you can and he is one of those players."
Earlier on Tuesday, Leeds announced the signing of Catalans half-back Richie Myler on a three-year deal starting next season.
Castleford Tigers have signed Leeds Rhinos and England international full-back Zak Hardaker for £150,000.