Japan Rugby League One is a rugby union competition in Japan. It is the highest level of professional rugby competition in the country. The Japan Rugby Football Union created the competition in 2003, by absorbing the Japan Company Rugby Football Championship, to drive up the overall standard and popularity of the sport and improve the results of the Japan national rugby union team. The chief architect of the league was Hiroaki Shukuzawa who strongly felt the urgency of improving Japanese domestic company rugby to a professional level which would allow Japan to compete more convincingly at Rugby World Cups.

Rugby union football, commonly known simply as rugby union or more often just rugby, is a close-contact team sport that originated at Rugby School in England in the first half of the 19th century. Rugby is simply based on running with the ball in hand. In its most common form, a game is played between two teams of 15 players each, using an oval-shaped ball on a rectangular field called a pitch. The field has H-shaped goalposts at both ends.