Germany's Federal Statistics Office said on Monday sales declined by 1.6 percent month-on-month in real terms last month, the biggest drop in seven months. On the year, sales were down 0.7 percent.
Economists polled by Reuters had forecast sales would rise by 0.3 percent month-on-month and 0.1 percent on the year. According to Reuters calculations retail sales declined 1.4 percent quarter-on-quarter in the July-September period.
Consumer spending accounts for around 60 percent of Germany's gross domestic product but weak private consumption has for years dogged Europe's largest economy, which relies heavily on exports for growth.
Alexander Koch, an economist at HVB Group in Munich, said sales had likely been hit by two key factors last month.
Since then energy prices have eased and the country's top two parties have neared a deal on forming a governing coalition.
Commerzbank economist Matthias Rubisch said the data pointed to a drop in private spending in the third quarter.
This assessment was backed up by the GfK market research group's forward-looking gauge of German consumer confidence, which last week pointed to an upturn in sentiment in November.
GfK said clarity on the political situation appeared to have boosted the consumer mood.
Oil prices have retreated from peaks of above $70 in late August, leading to a fall in fuel prices this month. However, Commerzbank's Rubisch said until Germany's ailing labour market picked up, there was unlikely to be any notable improvement in consumer spending.