Foster, 22, spent the second part of last season on loan at Hull FC, and now exits Langtree Park on a permanent basis after obtaining a release from his contract.
He said: “I am really looking forward to joining the Bulls and working with Francis Cummins. I know I’m a good player and I know I can score tries and kick goals and I’m looking forward to getting my head down and producing the form that I know I’m capable of.
“Working with new people will be good and hopefully bring out the best in me. When i got the opportunity at Hull, it was a case of kicking on again and hopefully the same thing can happen again here at Bradford.”
Foster made his St Helens debut back in 2010, going on to score 32 tries and 224 goals in 50 appearances for the club.
Bulls Head Coach Francis Cummins said: “Jamie has got some great experience already for such a young man but he’s also got to prove a few points here. I think he did that at Hull and he needs to kick on again now. It will be hard for him as well, initially, to leave Saints after such a long time but he’s got to take the next step in his career, as a lot of our players have had to do.”