Winterstein, 26, joined the Vikings at the start of 2012 from Crusaders, and has made 50 appearances, scoring 16 tries.
Coach Denis Betts, said: “Frank has been a valued member of our squad over the course of the past two seasons. Away from the field, he has also been a great ambassador for the club in the community.
“On behalf of the club and its supporters, I would like to thank him for his time, effort and energies spent in a Vikings shirt. Whilst doing so, I would also like to wish him the best of luck for his future.”