The benchmark Korea Composite Stock Price Index lost 8.92 points, or 0.38 percent, to 2,359.47.
Foreigners and retail investors offloaded shares worth more than 286 billion won ($250 million) and 168 billion won, respectively, while institutions scooped up more than 429 billion won, keeping the main index from falling further.
Ryu Yong-seok, a market analyst at KB Securities Co., said tensions between North Korea and the United States are a negative factor in the local stock markets that dragged down the index.
Park Chun-young, an analyst at Daishin Securities, also said North Korea's latest threat against the US weighed on the main index.
Earlier in the day, North Korea's military said it is considering a plan to fire four intermediate-range ballistic missiles around the US Pacific territory of Guam to "interdict the enemy forces on major military bases on Guam and to signal a crucial warning to the US"
The North's latest threat came as US President Donald Trump warned Tuesday that North Korea will be met with "fire and fury"
should it stick to ambitions to hit the US with ballistic missiles.
Most large-cap stocks were mixed across the board.
Auto parts maker Hyundai Mobis Co. was up 1.78 percent to 257,500 won, and No. 2 chipmaker SK hynix Inc. rose 0.31 percent to 64,400 won.
Meanwhile, market bellwether Samsung Electronics Co. fell 0.82 percent to 2,295,000 won, and top automaker Hyundai Motor was down 0.68 percent to 145,000 won.
The local currency closed at 1,142.00 won against the US dollar, down 6.80 won from the previous session's close.
Bond prices, which move inversely to yields, closed higher. The yield on three-year Treasurys fell 0.9 basis point to 1.824 percent and the return on benchmark five-year government bonds shed 1.4 basis points to 2.024 percent. (Yonhap)