The 30-year-old former New Zealand and Samoa international’s initial two-year contract was due to run out at the end of the season, but he has agreed a new deal that will keep him at the club until at least the end of 2019.
Castleford coach Daryl Powell said: “It’s fantastic news. Benny is a high-quality player and person who has got better and better as he has fitted into the way that we play.
“His game is constantly improving and I think there is more to come from him as he continues to strive for excellence and development in all aspects of his game.
“He fits the type of player we want at the club with his professionalism, his desire to win things and his passion to always be at his best every week.”
Roberts played in the NRL for Melbourne Storm, Parramatta Eels and Canterbury Bulldogs before moving to First Utility Super League and made 25 appearances in his first season.