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.
"We would also like to thank Penrith Panthers for allowing Luke to be released from his contract to join us next season."