The drop, to 125,096 units, brought year-to-date sales to 1,184,923, down 2.1 percent from last year's record nine-month pace.
Light-truck sales were off 16.6 percent for the month, while car sales slid 9.4 percent.
The Big 3, which have suffered at the hands of imports all year, registered the biggest hits, while several import brands posted gains.
Here are figures among the Big 3:
The two principal Korean carmakers, Hyundai and Kia, continued to gain momentum in the month.
Hyundai was up 44.9 percent in September, bringing its year-to-date gain to 54.2 percent, while Kia was up 36 percent in the month and 113.3 percent for the nine months.