The 26-year-old was perviously without a club, having left Hull FC during this season.
Saints coach Nathan Brown is delighted to have signed an experienced full-back, with Jonny Lomax sidelined for the rest of the season.
“Shannon is a great addition for us,” said Brown.
“He has a lot of experience in his position and will add strength and depth to our squad.
“If he wasn’t available or we needed him to play elsewhere then we don’t have anyone with that amount of experience to play in one of the toughest positions in the game.
“Bringing in Shannon means we have someone with a lot of experience to fill that role.
“He has already joined up with our squad and if everything goes well then he will play a game with one of our partnership clubs next week. We can then see where he is at.
“This is a great deal for us and a bonus for Shannon as he can play at a club that is doing well and showcase his talent to Super League and the rest of the world.”
“I’m pleased to be part of this club,” the player said.
“”It has great traditions, coaching staff and players and this is a good opportunity for me to come and play rugby league again.
“I am keen to get training and playing and I will get my head down and take each game as it comes.
“I like to play off the cuff and be consistent in both defence and attack. I am a full back but I will stick my hand up to play wherever the coach wants me to.”