The 26-year-old has played over 100 Super League games for the Tigers, after joining them as a 15-year-old.
“I’m over the moon to finally have this deal sorted and in the bag,” he said.
“There have been lots of rumours about me leaving the club but that has never crossed my mind. Despite having interest from here and overseas Cas is the place I want to be at.
“I’m extremely proud to play for this club and I’m sure in the next few years silverware will be coming our way. I’m excited at the prospect of the coming season and can’t wait to get stuck into pre season.”
Tigers head coach Daryl Powell is pleased that the forward has commited to the club.
“I’m delighted that Nathan has extended his deal at the club,” he said.
“I think it is clear to see the way that he plays that he loves playing at Cas. His drive and determination as a player will be massive for us moving forward into next season and beyond.
“Nathan is hugely respected by everyone in the changing room and on the coaching staff and this deal reflects the high regard we have for him.”