The volatile indicator, which is often subject to revision, showed retail sales in Europe's largest economy decreased by 0.4 percent on the month in real terms, the Federal Statistics Office said.
That compared with the Reuters consensus forecast for a 0.5 percent rise and followed a 1.2 percent drop in July.
On the year, retail sales jumped by 2.8 percent, matching the previous month's increase but undershooting a Reuters consensus forecast for an increase of 3.2 percent.
Traditionally thrifty Germans have helped private consumption displace exports as the main driver of growth thanks to record-high employment, increased job security, rising real wages and ultra-low borrowing costs.
The retails data comes after a GfK survey published on Thursday showed the cheerful mood among German shoppers clouding unexpectedly heading into October.
But the outlook for the economy remains bright overall, with institutes on Thursday hiking their growth forecasts for this year and next to 1.9 percent and 2 percent respectively.
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