This year's average of 28,168 was baseball's lowest since 1996 and was down from 30,012 last year, according to figures compiled by the commissioner's office.
Talk of a possible strike in July and August may have contributed to a decline that worsened in the last six weeks of the season.
After drawing a record average of 31,612 in 1994 before the start of a 232-day strike, baseball had a 20.1 percent drop the following year to 25,260.
Fans gradually came back, with the average rising each season from 1996-98.
Twenty of the 30 teams had declines this year. The largest loss was by the Milwaukee Brewers, which saw attendance decline by 841,888, 30 percent of 2001's team-record 2,811,041.
Pittsburgh had the second-biggest decline, a 651,456 drop from 2,436,126.
Seattle, despite missing the playoffs, drew a major-league high 3,540,658, an average of 43,712. The New York Yankees were second, drawing a team-record 3,465,807, an average of 43,323.
San Francisco led the National League at 3,253,203, an average of 40,163, followed by World Series champion Arizona at 3,198,985, an average of 39,494. The Diamondbacks had the largest increase, a rise of 458,431.