The benchmark Korea Composite Stock Price Index dropped 6.80 points, or 0.29 percent, to 2,319.82, extending its losing streak to a fifth day. Trade volume was low at 249 million shares worth 5.25 trillion won ($4.62 billion), with losers outnumbering gainers at 569 to 239.
Analysts said Wednesday's decline came as the overnight losses on Wall Street and North Korea's nuclear test prodded investors to seek safer assets.
Experts, however, said the geopolitical factors may have short-lived impact on the local financial market.
"The KOSPI kicked off lower on the sluggish US market. The decline, however, was limited on eased geopolitical risks," said Seo Sang-young, a researcher at Kiwoom Securities Co.
Foreigners offloaded a net 325.5 billion won, while individual investors scooped up a net 107 billion won. Institutions bought more shares than they sold at 175 billion won.
Carmakers finished lower, with industry leader Hyundai Motor moving down 1.45 percent and its smaller sister Kia Motors falling 3.67 percent. Auto parts arm Hyundai Mobis stepped down 1.47 percent.
No. 1 steelmaker POSCO slipped 2.19 percent to 335,000 won, and Korea Zinc closed unchanged at 517,000 won. No. 2 player Hyundai Steel slid 2.63 percent to 55,500 won.
Kumho Tire nosedived 18.69 percent after its creditors decided to reject a demand by China's Qingdao Doublestar to cut its price for a controlling stake in the South Korean tiremaker.
Leading mobile carrier SK Telecom decreased 1.61 percent to 244,000 won, and No. 2 player KT moved down 0.99 percent to 30,050 won. LG Uplus, the smallest player, shed 1.82 percent to 13,500 won.
Tech shares, on the other hand, closed higher. Market behemoth Samsung Electronics rose 0.51 percent to 2,350,000 won, and No. 2 chipmaker SK hynix also added 2.58 percent to 71,700 won.
The local currency closed at 1,135.40 won against the US dollar, down 4.30 won from the previous session's close.
Bond prices, which move inversely to yields, ended higher. The yield on three-year Treasurys shed 0.5 basis point at 1.775 percent and the return on the benchmark five-year government bonds also lost 1.3 basis point to 1.983 percent. (Yonhap)