Saints have signed Theo Fages from Salford Red Devils.
The 21-year-old is regarded as one of the most exciting young prospects in the game.
Operating predominately at scrum-half, he can also play at hooker and stand-off.
He joins the defending Super League champions on a four-year contract.
Head coach Keiron Cunningham said: “Theo is a super player and I am really pleased to add him to our squad for 2016. He’s talented, can play in a number of positions but is also young and keen to learn.
“He will fit into our systems perfectly and we’re delighted to secure his services going forward.”
Fages joined Salford at 16 and progressed through their Academy system to make his debut against Wigan in 2013.
He has since gone on to make more than 60 appearances for the Red Devils, chalking up more than 20 tries.
A French international too, he played in the 2013 Rugby League World Cup and also the European Cup the following year.