The half back will join from Penrith Panthers and has the option to extend his stay by a further year.
At 26-years-old he's played 107 games in the NRL for both Newcastle and his current club - amassing 274 points.
Saints Head Coach Nathan Brown said: "Luke is an exciting addition to the squad. He is a good organiser and possesses a great kicking game too.
"We are always looking to improve the team and I'm sure Luke will contribute to that. We are looking forward to having him on board.