The 21-year-old, who was born in Pontefract and is a product of the Normanton Knights community club, scored eight tries in 22 first team appearances for Castleford.
He could make his competitive debut against Tigers, who play host to Leigh in the opening round of Super League 2017.
Leigh coach Neil Jukes said: “Ryan wants to challenge for a first-team spot and can play half-back, full-back or wing and do a good job in all departments.
“First and foremost his aim is to claim a place in the starting 17 in one of those spots.
“I remember him coming off the bench against us in a pre-season friendly and he made an impact straight away.
“He has energy, sharpness and an eye for an opportunity and he will bring freshness to our squad.”
Jukes went on: “We have added some youth lately and Ryan’s signing enhances that energy in the squad.
“He is spoken of very highly and having come through the Wigan system has had a great grounding in the game.
“Ryan will get his opportunity in the off-season to stake his claim and he will push a lot of other players for a back-line spot.
“We know that next season will be a tough one and we need strength in depth in every position. From a coach’s perspective the fact that he offers such versatility is an added bonus.”