Some 57 percent said Lee is doing a good job, down 7 percentage points from a similar survey conducted just before the June 3 local elections.
After months of strong approval ratings since early February, his support slipped below 60 percent for the first time.
Some 35 percent disapproved of him, while the remaining 8 percent were undecided or declined to answer.
Gallup Korea attributed the decline in his support to disruptions during early this month's local elections, in which some voters were unable to cast ballots due to shortages in ballot papers, among other reasons.
In fact, among those who disapproved, the top reason was election-related irregularities and poor management, cited by 16 percent, followed by economic concerns at 14 percent and dissatisfaction with real estate-related policies at 9 percent.
Meanwhile, support for the ruling Democratic Party (DP) also dropped by about 4 percentage points to 41 percent, while that of the main opposition People Power Party (PPP) rose by 7 percentage points to 29 percent.
Other minor parties including the centrist Reform Party (RP), the liberal Rebuilding Korea Party (RKP), and the left-wing Progressive Party each had 2 percent support, while 20 percent supported none of the parties.
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