The Founding Era - Loose Roster Construction Through the Postwar Period
From NPB's 1937 founding through the postwar years, roster regulation was looser than today. Team sizes were smaller, and most franchises operated with about 30 registered players. Hard caps did not exist; clubs largely used their own judgment. The wartime suspension and revival of pro baseball, plus the 1950 league split that doubled the number of teams, created chronic shortages of players. In this period, roster debates focused less on competitive balance than on the basic problem of supplying enough players. For decades after the founding, roster limits were not the structural lever they would become.
From 25-Man to 28-Man Active Rosters
Over time NPB formalized roster regulations. The longstanding system allowed franchises to keep about 70 players on the controlled list, register 28 to the active roster, and dress 25 for any given game. The 25-player game-day limit shaped bench construction. By restricting how many bench players, pinch hitters, and relievers could be available on a given night, the rule forced franchises to balance roster mix carefully. The three-player gap between the 28 active and 25 dressed offered slack for conditioning and travel. This framework operated steadily for roughly three decades.
September Roster Expansion - Late-Season Flexibility
NPB also has long permitted roster expansion from September 1 through the end of the regular season, a parallel to MLB's September roster. The structure lets contending teams add depth as the pennant race climaxes. During expansion, active rosters have been allowed to swell to 31 or 32 players, often used to showcase young talent and stretch the bullpen. Many young players who debuted in September became core contributors the following year, making expansion a key venue for talent discovery. Late-season managerial decisions hinge on how a manager deploys these extra arms.
How COVID-19 Changed 2020 - The Birth of the 31-Man Roster
The 2020 season opened late and ran on a compressed schedule due to COVID-19. Under those conditions, NPB temporarily expanded the active roster. Handling positive cases, isolating close contacts, and using PCR results to make availability decisions overran the old 25-man framework. When the season opened in June 2020, NPB lifted the active roster to 31. The change was initially emergency relief, but franchises lobbied for permanence. The 31-man roster broadened bench flexibility, especially for bullpen deployment. With pitch-count restrictions tightening, carrying seven or eight relievers gave managers room to manage workloads.
The 31-Man Roster Settles In and Reshapes Tactics
Even after pandemic conditions eased, NPB retained the 31-man roster. The larger active list spawned several tactical shifts. First, bullpen specialization deepened: starters, middle relievers, setup men, and closers expanded into one-out specialists, mop-up pitchers, and long relievers. Second, defensive replacements and pinch runners earned dedicated slots, elevating the value of speed and glove specialists. Third, young players gained more chances on the active roster, easing generational handoffs. The 31-man roster has been a qualitative reform to NPB's tactical layer, not just a quantitative tweak.
Comparison with MLB and Ongoing Debate
MLB operates dual rosters: 26 in regular season and 28 during September expansion. NPB's 31 is larger, producing tactical divergence. MLB relievers typically take full days off between outings, while NPB clubs can carry deeper bullpens, supporting strict workload spacing alongside specialized win-pattern relays. Discussion continues over whether to expand further or return to the 28-man baseline. The 31-man roster has raised payroll burdens for bench depth, and team finances factor into the debate. NPB's roster history is a layered design problem that balances competitive equity, cost structure, and tactical deployment.