The 33-year-old Samoa international played 78 matches for the Tigers after joining them from Melbourne in 2015 and was a member of their Grand Final team in 2017 but has been restricted to one solitary substitute appearance in 2019 due to injury.
Castleford coach Daryl Powell said: “I think it’s fair to say it’s been frustrating for both Benny and ourselves that he hasn’t been able to get on the field as often as he’d like and unfortunately that looked increasingly unlikely.
“We therefore felt it was best to come to an agreement between ourselves so that we could all move forward.
“Benny has had some fantastic times at the club, particularly in 2017 when he played probably the best rugby league of his career.
“It’s been tough but it’s an amicable agreement and I’d just like to wish Benny and his family all the best for the future.”
Roberts said: “First and foremost, I want to thank God for giving me and my family this opportunity.
“To the fans who have supported me and the boys for the last four years, I would like to thank you for making the experience of playing for the Tigers one that me and my family will never forget.
“To my team-mates who I see as life-long friends, I wish you all the success for the rest of the season and beyond.”