In a move that could potentially undermine baseball's true World Series, Japan's pro baseball players union threatened to boycott next year's World Baseball Classic. The Japanese pro players want a greater share of the tournament's revenue from the event's organizers—a joint operation between Major League Baseball and its players' association.

Major League Baseball started the WBC in 2006 as the sport's answer to soccer's World Cup and to help spread the game's popularity globally. If Japan's pro players, who have comprised...