The business sentiment index (BSI) came in at 69 in January, down 3 points from the previous month, according to the Bank of Korea (BOK).
It was the lowest since March 2016. The reading below 100 indicates pessimists outnumbering optimists.
The fall came as export of semiconductor, which led the country's overall export for the past year, turned downward. Chip export tumbled 28.8 percent in the Jan. 1-20 period from a year earlier.
Hit by the weak chip export, the BSI for electronic and communication equipment companies retreated 8 points to 70 in January. It was the lowest since June 2016.
The BSI among manufacturers shed 4 points over the month to 67 in January, and the figure for non-manufacturers fell 2 points to 71 in the month.