The West Yorkshire club tonight confirmed they have captured the France international on a three-year deal.
Fages, 27, has developed into one of Super League's top half-backs since joining from Salford Red Devils in 2016.
He has helped them secure three successive titles and played in the 2019 and 2020 Grand Finals although missed Saturday's latest success having broken a shoulder in July's Challenge Cup final win over Castleford Tigers.
With Aidan Sezer and Lee Gaskell both having left Giants, Fages is set to link up with fellow new signing Tui Lolohea in the halves at John Smith's Stadium next term.
With more than 200 Super League games, he was also captain of France in the last World Cup and has 15 caps.
“Theo is a hugely significant signing for us at the Giants," said Watson, who has also added England prop Chris Hill and Brisbane Broncos hooker Danny Levi to his rosta for 2022.
"He’s a player with a huge desire to win and be the best, a never-say-die attitude who demands the team to compete hard on every play by leading with his actions.
"Theo has helped take a Saints team to the very top with his desire and ability to control the game through his organisation and kicking abilities.
“Theo, along with the other players in the squad, can now build a team which pushes us to a new level.”
Fages said “I’m really excited to get going.
"I just want to bring this team back to where it was - fighting for trophies - and the challenge is really exciting for me.
"It’s a new life for me, we will be moving here and I’m really looking forward to it.
"Ian Watson and everybody else wants to do really well for this team. We want to win trophies and I’ll just be giving 100% for this team."