Hull FC second-rower, Willie Manu will join St Helens at the end of the season on a two year contract.
The Tongan international rejected numerous new contract offers from the Airlie Birds to become Nathan Brown's first signing for 2013.
The 32 year-old, who is currently in the Exiles camp preparing for Saturday's clash against England, rejected a move to Catalan in favour of linking up with Brown.
“Willie is one of the best back-row forwards in rugby league and will introduce a new dimension to our game,” said St Helens chairman Eamonn McManus.
“He is a devastating runner with a phenomenal workrate 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.”