The benchmark Korea Composite Stock Price Index lost 8.31 points, or 0.35 percent, to 2,380.40.
Foreign investors offloaded shares worth more than 33.6 billion won ($29.6 million) and institutions sold more than 47.8 billion won.
Park Chun-young, an analyst at Daishin Securities, said the decline is blamed on the tensions between North Korea and the US, and the profit-taking ahead of a long holiday next week.
South Korea's stock markets will be closed from Oct. 2-9 as people celebrate the Chuseok holiday -- the equivalent of Thanksgiving in the US
Kim Ye-eun, an analyst at Cape Investment & Securities, also said institutions sold large-cap stocks in apparent profit-taking, which are to blame for losses on the main bourse.
Most large-cap stocks were mixed across the board.
Market bellwether Samsung Electronics Co. gained 1.17 percent to 2,681,000 won, and No. 2 chipmaker SK hynix Inc. was up 3.85 percent to 86,300 won.
Meanwhile, top automaker Hyundai Motor fell 0.70 percent to 141,000 won, and Naver, the operator of the country's top Internet portal, lost 1.31 percent to 754,000 won.
The local currency closed at 1,131.80 won against the US dollar, up 4.70 won from the previous session's close.
Bond prices, which move inversely to yields, ended lower. The yield on three-year Treasurys gained 1.1 basis point to 1.786 percent and the return on the benchmark five-year government bonds rose 1.0 basis point to 1.982 percent. (Yonhap)