The 18-year-old will be playing in the All Japan High School Soccer Tournament, representing Chukuodai Chuyko High School, and will join when the championships have concluded.
"I am delighted Ryo will be joining us in the New Year," Arsene Wenger told the club's website.
"He trialled with us in the summer and has raw ability which has attracted many clubs around the world.
"I look forward to helping him fulfill his potential here at Arsenal."
