The month saw 462,517 cars registered, up 8.6% on last year, said the Society of Motor Manufacturers and Traders.
More than two million cars have been registered so far this year - the first time that the two million mark has been passed in September since 2004.
Car sales have now increased for 43 months in a row, the SMMT said.
March this year saw the the highest monthly level of new car sales this century.
SMMT chief executive Mike Hawes said: "September is traditionally one of the year's biggest months for new car registrations, and last month set an autumn record."
Changes to number plates in September means the month is traditionally one of the busiest of the year.
A spokesman for SMMT said it was too early to say what kind of impact VW's emissions scandal could have on sales.
BBC industry correspondent John Moylan said there were "signs of a growing gap" in the market share of diesel and petrol cars.
"Last year consumers bought more diesel cars than petrol. But in September, 45.9% of car sales were diesel and 51.4% were petrol."
He added that the Volkswagen scandal, with its focus on diesel cars, "is unlikely to help" future diesel sales.
1. Ford Fiesta - 22,807
2. Vauxhall Corsa - 15,670
3. Ford Focus - 14,258
4. Volkswagen Golf - 13,603
5. Volkswagen Polo - 11,026
6. Nissan Qashqai - 10,119
7. Mini - 8,754
8. Vauxhall Astra - 7,920
9, Vauxhall Mokka - 7,670
10. Fiat 500 - 7,460
UK new sales have registered their highest level for September on record, with gains in private, fleet and business sales.