The falloff marked the 10th-consecutive month of declining sales as the gaming world holds off buying ahead of the release of Nintendo's Wii U console next month
Research firm NPD Group said Thursday that sales fell to $848 million from $1.11 billion a year earlier.
Sales of video games themselves, excluding PC titles, fell 18 percent to $497.4 million.
The September decline was less severe, however, than the 28 percent drop forecast by Cowen & Co. analyst Doug Creutz.
Including PC games, software sales fell 14 percent to $547.3 million. That was also better than the 23 percent decline Creutz forecast. The analyst said he expected the sales declines to moderate somewhat in the final quarter of the year due to new releases and Nintendo's Wii U, which is due out Nov. 18.
September sales of hardware such as the Xbox 360 fell 39 percent to $210.9 million, and accessories sales fell 11 percent to $139.9 million.
The best-selling title of the month was Electronic Arts Inc.'s "Madden NFL 13," followed by Take 2 Interactive Software Inc.'s, "Borderlands 2" and EA's "FIFA Soccer 13."