The composite consumer sentiment index (CCSI) stood at 97.9 in May, down 3.7 points from the previous month, according to the Bank of Korea (BOK).
It was the first decline since November last year. The reading below 100 means pessimists outnumbering optimists.
The worsened consumer confidence came as export, which accounts for about half of the export-driven economy, continued to slide for five months through April.
The country's real gross domestic product (GDP), adjusted for inflation, slipped 0.3 percent in the first quarter from the previous quarter.
Sub-indices all lost ground. The CSI for current economic situations shrank 5 points, and the figure for prospective economic conditions contracted 6 points in the month.
The CSI for prospective household income fell 2 points, and the reading for prospective consumer spending slipped 1 point. The figure for prospective job opportunity declined 3 points this month.
Inflation expectations, which reflect outlook among consumers over headline inflation for the next 12 months, stood at 2.2 percent in May, up 0.1 percentage point from a month earlier.
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