However, retailers remained confident that sales in the last three months of 2011 would increase due to spending for Christmas and New Year celebrations, says the survey of 290 retailers in four major cities.
Retail sales index in September rose 18.1 percent year-on-year, lower than 33.5 percent in August, according to the survey that uses October 2000 as the base period.
Retailers also expected lending rates to decline in the next three to six months, the survey said.
Retail sales account for nearly 60 percent of household consumption in an economy seen expanding 6.6 percent this year, despite the turmoil in global financial markets. The economy grew by an annual rate of 6.5 percent in the first half this year.
Indonesia's central bank cut its benchmark rate in October by 25 basis points to 6.5 percent to stimulate the domestic economy as global growth slows.