The 29-year-old will join the club next year after being a solid performer for the toiling Bulldogs for the past six seasons.
The Bulldog will have the option to extend for a further season after the contract expires should he wish to play on, meaning Hopoate could stay with St Helens until the end of the 2024 season.
The Tongan international made his NRL debut back in 2010 for Manly before making a switch to Parramatta in 2012 and then finally to the Bulldogs in 2016.
Hopoate featured five times in the NSW Origin squad from 2011 to 2015 and captained the Tongan side in 2017 and 2018.
The centre, who is also just as suited to playing fullback, expressed his delight at being able to join St. Helens.
“I am very happy and excited to be making the move to St Helens and to play for such a proud and historic club," Hopoate said to the club's website.
“I have been coached by ‘Woolfie’ before for Tonga and that was a big reason why I made the decision. I know him as a person and as a coach and his values and standards that he sets is something I want to buy in to."
Hopoate also revealed he had offers from multiple NRL clubs on the table but turned them down in search of a new challenge.
It was rumoured that the 29-year-old might have been part of a swap deal between St George Illawarra and Canterbury, but an approach from St. Helens was too much for him to resist.
“I had option to sign for Club’s in the NRL, but I have always wanted to play in the Super League and a strong Club like St Helens meant I just couldn’t turn it down."
“If I go back to 2019 and Tonga played against Great Britain and Australia and he was one of our best players against two top quality teams on the world stage which gives you an idea of the quality we are getting.
“From a personal point of view, he is not just a good player, but is a great bloke and will fit into the community and Club really well.”
Hopoate will want to finish off his career in the NRL strongly, with his side rock bottom with four rounds to go in season 2021.
This week they face the Warriors in Redcliffe, with a win here potentially giving the side hope they can still move off the bottom of the ladder before season's end.