Hargreaves, 24, started his career at Wigan and will follow incoming coach Mick Potter across the Pennines from Knowsley Road.
The prop has signed a two year deal at Odsal, having played more than 100 games in a four year spell with Saints.
He said: “When I spoke to Lee St Hilaire about joining Bradford I was impressed with what I heard and I really think the club’s going places. Their current position is not reflective of where they are right now and I think they will soon be back where they belong. As far as I’m concerned, it’s time for a change and for a new challenge. I’ve enjoyed quite a bit of success at St Helens and now I’m hopeful of doing the same at Bradford.”
He added: “Bryn will be a tremendous addition to our Squad next season. He is still very young and his best years are very much ahead of him but he has a wealth of experience through being virtually ever present in a very good St Helens side for the past two years. We are delighted he has chosen to come to the Bulls and he will complement our other Prop Forwards immensley.”