Winterstein, 26, has spent the past two seasons with the Vikings, and has also played in Super League for Wakefield and Crusaders.
He said: “I’ve met Franny, his coaching team and all the backroom staff. I was really impressed with all the facilities at Tong and what their vision is to go forward.
“They will definitely have a good squad next year and I’m looking forward to moving over.
“It’s a great club and hopefully we can bring it back to its glory days. It’s about gelling as a team and really pushing hard for a play-off spot.”
Winterstein made 23 appearances for Widnes in 2013, scoring nine tries.
Bulls coach Francis Cummins added: “Frank is a great signing for us. He is big and powerful but he has great skills and can play in a few positions.
“I was very impressed with him when I met him and I am sure he will prove to be a valuable member of our squad.”