The benchmark Korea Composite Stock Price Index shed 16.47 points, or 0.7 percent, to 2,341.22 as of 11:20 a.m.
On Sunday, Pyongyang announced it had successfully conducted a test of a hydrogen bomb that can be mounted on an intercontinental ballistic missile.
Market behemoth Samsung Electronics shed 0.56 percent, and top chipmaker SK hynix slid 1.02 percent. LG Electronics moved down 0.24 percent.
Chemical shares were also losers, with AmorePacific falling 0.17 percent and LG Household & Health Care decreasing 0.73 percent. LG Chem lost 0.4 percent.
Leading carmaker Hyundai Motor and its sister Kia Motors lost 1.05 percent and 0.56 percent, respectively.
The local currency was trading at 1,131.15 won against the US dollar, down 8.35 won from the previous session's close. (Yonhap)