Food sales were down 0.6% compared with August in data adjusted for the number of working days and seasonal factors. However, fuel sales were up 4.7% on the month, while sales of non-food items were 2.4% higher compared with August.
Compared with September 2012, adjusted retail sales declined 0.8%, again due to lower food sales, INS data showed.
Sales of food, drink and tobacco products were down 3.8% on the year in September, while sales of non-food products rose 5.5% and fuel sales edged 0.4% higher.
In the first nine months of 2013, adjusted retail sales were down 0.3% compared with the year-earlier period, the data showed.
In unadjusted data, retail sales were down 2.4% in September from a month earlier, but up 1.1% compared with the same period of 2012.