The figure represents 15% of the total US adult population.
A further survey found 39% of self-described "avid" fans plan to place a bet on the NFL this season.
With 13 states now hosting legal, regulated sports betting markets and others expected to follow, the AGA found 24% of US adults would bet on the NFL if it was legal and convenient to do so in their state.
The results suggest the legalization of sports betting has increased the number of Americans who will bet legally, with the total of those saying they will place bets at casino sportsbooks rising 19% from last year, to 6.9 million people.
The AGA estimates the NFL will make approximately $2.3bn per year from a widespread legal sports betting market, due to increased fan engagement.
Further research suggests placing wagers makes fans more likely to engage with the NFL, with 75% of bettors saying they are more likely to watch a game they have bet on; 28% said they are more likely to attend a game they have bet on.
