Winterstein, who spent the last two years with Widnes after previous stints in Super League at Wakefield and Crusaders, has signed a 12-month contract and will link up with the Bulls after completing his World Cup commitments.
Bradford coach Francis Cummins said: "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."
The 26-year-old Winterstein, who won his eighth cap for Samoa in Sunday's 42-24 defeat by New Zealand in Warrington, becomes Cummins' sixth signing for 2014 following the capture of Garreth Carvell (Warrington), Luke George and Dale Ferguson (both Huddersfield), Lee Gaskell (Salford) and Adam Henry (Sydney Roosters).
"The Bulls have brought in some good signings and kept a couple of guys who were on loan there," Winterstein said. "They will definitely have a good squad next year and I'm looking forward to moving over.
"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.
"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."