About 73.8 percent of Greece's 1.11 million jobless are long-term unemployed, meaning they have been out of work for at least 12 months, the ELSTAT figures showed.
Young Greeks aged 15-24 remain the hardest hit, with unemployment at 44.2 percent in the July to September period, slightly lower compared to 48.8 percent in the same quarter a year ago.
Greece's debt crisis and austerity-induced recession have wiped out about a quarter of the country's economic output and drove the jobless rate to the highest in the euro zone.
It hit a record high in the first quarter of 2014, at 27.8 percent.
Athens has already published monthly unemployment figures through June, which differ from quarterly data because they are based on different samples and are seasonally adjusted. Quarterly figures are not seasonally adjusted.