St Helens have won the race to sign Willie Manu on a two-year deal from 2013.
The second-row forward has been at Hull for the last six seasons and has been linked with a host of clubs with his current contract running out at the end of the campaign.
"Willie is one of the best back row forwards in Rugby League and will introduce a new dimension to our game," St Helens chairman Eamonn McManus told the club's official website.
"He is a devastating runner with a phenomenal work rate and is one of the league's top tackle bust makers. He is also a great defender too and we look forward to welcoming him next season."