Researchers at the Korea Development Institute say weak demand domestically appears to have caused overall economic activity to stagnate.
According to KDI's monthly report released on Thursday, exports increased, but domestic demand remained weak, which held back investment and consumption.
Back in August, KDI's report said the economy was "improving," but it did not use that language in the two months after that.
According to the latest report, investment in facilities decreased sharply in September, especially in the machinery sector, by almost 20 percent.
That was mainly because of fewer working days for the month.
Facilities investment had fallen in August, too, but by only 11 percent.
More grim numbers came from consumption.
Retail sales in September increased on year by only half a percent compared to a gain the previous month of almost 6 percent.
The durable goods sector, in particular, fell by almost 10 percent because of poor sales of passenger cars.
September's industrial production also recorded negative growth, but the institute says that was only a one-off dip because of a holiday.
There were some uplifting numbers in exports thanks to semiconductors.
October's exports were up more than 22 percent on-year.
Breaking it down by item, semiconductors posted more than 20 percent growth, but the nation's shipbuilding industry continued to struggle, marking a plunge in exports of more than 50 percent.
Earlier this week KDI lowered its growth forecast for the Korean economy for this year from 2.9 percent to 2.7 percent.
Ko Roon-hee, Arirang News.