The highest attendance for Major League Baseball (USA)during a single-season is 73,070,631 in 2005; an average of 30,936 per game.

The previous record was for the 2004 season, with 73,022,969 (or an average of 30,401 per game).

NB 2005 figures released in September – four days before the end of the ‘regular’ season.

Source:
MLB sets single-season attendance record
Baseball in ‘golden era,’ breaks mark for second straight season
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