The benchmark Korea Composite Stock Price Index shed 11.94 points, or 0.5 percent, to 2,358.36 in the first 15 minutes of trading.
North Korea lobbed a ballistic missile over Japan earlier in the day in a move that can sharply escalate tensions in Northeast Asia.
Market kingpin Samsung Electronics was 1.08 percent down, while SK hynix, a major chipmaker, sank 0.74 percent.
Hyundai Motor, the No. 1 automaker, moved down 0.69 percent, while AmorePacific, the country's top cosmetics maker, gained 0.17 percent.
POSCO, the No.1 steelmaker, advanced 1.35 percent.
The local currency was trading at 1,127.65 won against the US dollar, down 7.55 won from the previous session's close. (Yonhap)