The French scum-half’s move to Saints was revealed on Twitter earlier on Thursday by Salford owner Dr Marwan Koukash.
Saints have agreed a transfer fee in line with the rugby league compensation rules set out for home-grown players.
The 21-year-old played 60 games for Salford, scoring over 20 tries.
“Theo is a super player and I am really pleased to add him to our squad for 2016,” said Saints coach Keiron Cunningham.
“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.”