The composite consumer sentiment index for September stood at 107.7, down 2.2 points from the previous month, according to the survey by the Bank of Korea.
In August, the index fell 1.3 points from July. A reading above 100 means optimists outnumber pessimists.
"The issue of North Korea's nuclear weapons program combined with the standoff over the US missile shield to make local consumers feel less optimistic this month," a BOK official said.
The United States and North Korea have been locked in a fiery war of words over Pyongyang's nuclear and missile programs, escalating tensions on the Korean Peninsula.
South Korea's relations with China, its largest trading partner, have remained strained due to Seoul's deployment of an advanced US anti-missile battery here, which Beijing sees as a security threat. China has taken a series of retaliatory measures against South Korea, including a ban of group tours, hitting South Korea's tourism industry and other China-focused firms hard.
The BOK also said its index measuring the people's sentiment toward the current economic conditions came to 87 in September, down from 93 the previous month.
In addition, an index measuring the public's feelings toward economic conditions in the next six months sank to 96 from 104 over the cited period.
It marks the first time in five months that the index on future economic conditions has fallen below the benchmark 100.
The latest monthly index is based on a survey of 2,013 households throughout the country from Sept. 12-19. (Yonhap)