The report says that consumer spending, excluding autos, fell 2.4 percent last month on a seasonally adjusted basis compared with a 0.4 percent increase in August. The information was provided by SpendingPulse, the retail data service of MasterCard Advisors.
The September reading was the biggest single-month drop in SpendingPulse history going back to 2003.
Consumers reined in spending on gasoline after the summer driving season, and this bogged down retail sales in September, said Kamalesh Rao, director of economic research at MasterCard Advisors, which is the professional services arm of MasterCard Worldwide.
"They spent a lot less on gasoline. That's a huge expense for a lot of people," Rao said.
Gasoline prices have fallen steadily after setting records in July. They averaged $3.48 a gallon last week, falling below $3.50 for the first time in six months but still up 71 cents from a year earlier, the government said on Monday.
Last month's pullback was broad-based with categories like electronics and furniture/home furnishings posting double-digit sales drops from a year ago, Rao said.
"We've seen dramatic declines in a lot of sectors," he said.
SpendingPulse's "core" measure on consumer spending, which factors out cars, gasoline and building materials, grew a measly 0.2 percent against a 0.9 percent rise in August.
Meanwhile, non-durable goods prices slipped during September, but they were not enough to offset the drop in demand.
SpendingPulse's index of non-durable goods prices was down 2.1 percent in September from August but still up 7.1 percent from a year ago.
The SpendingPulse data are derived from the aggregate sales in the MasterCard U.S. payment network, coupled with estimates on all other payment methods including cash and check. They do not represent MasterCard's financial performance.