Interleague play , 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).

The 2004 Nippon Professional Baseball realignment was a series of events that occurred during the 2004 Nippon Professional Baseball season that changed the landscape of Nippon Professional Baseball (NPB). In June of that season, the Osaka Kintetsu Buffaloes and the Orix BlueWave announced that, due to financial difficulties, the two teams planned to merge into one for the start of the 2005 season. Both teams were in the Pacific League (PL), and a merger between the two would result in a team imbalance with the PL's opposing league, the Central League (CL). Soon, it was announced that a second merger was being explored between two of the remaining four PL teams. With the possibility of the PL losing a second team, discussion about possibly restructuring NPB's two-league system into one ten-team league began. PL and CL executives continued to discuss the merits of both systems until it was finally decided that the two-league system would remain intact and interleague play would be introduced in the 2005 season.