The reigning Super League champions have paid an undisclosed fee for the 21-year-old, who leaves the Salford Red Devils.
St Helens coach Keiron Cunningham told the club website: "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, who can play at hooker or half-back, will take up his contract on December 1.