Exports by Asia's fourth-largest economy totalled $25.70 billion during the Aug 1-20 period while imports for the same period rose 22.4 percent from a year earlier to $30.46 billion, Korea Customs Service data showed.
The agency did not provide comparable figures for 2010.
Import growth for the 20-day period also slowed from a 30.5 percent rise during July 1-20, although the trade deficit widened to $4.76 billion in August from a $0.40 billion deficit posted in July.
South Korea's foreign trade figures for the full month could still change significantly but investors and analysts closely watch the interim readings for clues on the overall strength in global demand.
South Korea has enjoyed robust exports from emerging-market economies and deliveries of ships, but worries about a possible double-dip slump in the United States has made the outlook uncertain.
Copyright Reuters, 2011