Interleague play (NPB)

Interleague play (セ・パ交流戦, Se・Pa kōryū-sen), officially titled Nippon Life Interleague Play for event sponsor Nippon Life, is an event consisting of 108 regular-season baseball games played between Central League (CL) and Pacific League (PL) teams in Nippon Professional Baseball (NPB).

Prior to 2005, matchups between CL and PL teams occurred only during spring training, the All-Star Series, a short-lived exhibition tournament called the Suntory Cup, and the Japan Series. Central League teams were reluctant to implement regular-season interleague play as it would reduce the money generated from games played against the Yomiuri Giants, the hugely popular CL team that generates the most money in Japanese baseball.

However, during the 2004 NPB realignment, the merger of two PL teams that were struggling financially, the rumor of a second PL team merger, and talks of contracting and restructuring the two-league system into one ten-team league prompted the suggestion of interleague play as a possible solution. Team representatives eventually approved one merger, agreed to maintain the two-league system, and approved to hold interleague regular-season games during the 2005 season.

Originally, each team played 36 games against teams in the other league. Two years later, interleague play was cut to 24 games per team for the 2007 season. The amount of interleague regular-season games was again reduced for the start of the 2015 season to 18 games per team where it has remained through today. Interleague play is a single event within the regular season with all games being played during a three-week block beginning in late May. Each CL and PL team plays a three-game series at home against half the teams in the opposing league and a three-game set on the road against the other half. Each consecutive season, teams switch these home-and-away roles. The designated hitter is implemented when PL clubs are the home team.

Since its implementation, the Pacific League has won more games against the Central League every year except three, the 2009, 2021, and 2022 seasons. At the conclusion of interleague play, the team with the best winning percentage during the event is named the winner; a PL team has come out on top every season but four. The event's sponsor also announces three player awards: one interleague Most Valuable Player (MVP) award and two Nippon Life Awards, one from each league.