The benchmark Korea Composite Stock Price Index lost 21.27 points, or 0.89 percent, to 2,373.46 as of 11:25 a.m.
The decline followed overnight losses on Wall Street. The Dow Jones industrial average lost 0.15 percent, with the tech-heavy Nasdaq composite index declining 0.21 percent.
Kim Ye-eun, an analyst at Cape Investment & Securities, said North Korea risks are to blame for the slight decline in the main index.
"Tensions between North Korea and the US are likely to have a negative impact on the main index," Kim said.
On Wednesday, North Korea's military said it is considering missile strikes near Guam, warning that it is ready to stage an all-out war if Washington launches a preventive war against it.
The North's threat came hours after US President Donald Trump warned that any threat by North Korea to the US will be met with "fire and fury."
Most large-cap stocks were mixed across the board.
Top automaker Hyundai Motor was up 0.69 percent, and top steelmaker POSCO advanced 0.44 percent.
Meanwhile, market bellwether Samsung Electronics Co. backtracked 2.05 percent, and No. 2 chipmaker SK hynix Inc. was down 2.56 percent.
The local currency was trading at 1,135.20 won against the US dollar, down 10.10 won from the previous session's close. (Yonhap)