Export, which takes up about half of the export-driven economy, tumbled 21.8 percent from a year earlier to 28.5 billion U.S. dollars in the September 1-20 period, according to Korea Customs Service.
The number of working days declined by two days in the cited period compared with a year ago as the Chuseok holiday fell on this month.
To exclude the effect of less business days, the daily average export diminished 10.3 percent to 2.11 billion dollars in the same period.
The country's export kept sliding for the ninth consecutive month through August.
Shipments for semiconductors, oil products and cars declined 39.8 percent, 20.4 percent and 16.6 percent each, but those for telecommunication devices such as smartphone advanced 58.0 percent in the 20-day period.
Export to China, South Korea's biggest trading partner, dropped 29.8 percent, with those to the United States, Japan and the European Union (EU) plunging in double figures.
Import retreated 11.1 percent to 26.9 billion dollars in the 20-day period, sending the trade surplus to 1.6 billion dollars.