The consumer price index was up 0.4 percent in April from a year earlier, unchanged from the rate in March, Statistics Korea said.
Rises in rents and services were largely offset by sharp declines in energy costs as oil prices plunged 20.9 percent from a year ago.
South Korea imports almost all of its energy needs.
Inflation has remained far below the Seoul central bank's target of 2.5 to 3.5 percent for nearly three years.
Despite record-low interest rates, South Korea's central bank predicted annual inflation would drop below the 1.0 percent mark in 2015, fuelling fears of deflation that could hammer an overgrown property market.
The exports that drive the country's economy have fallen in every month this year, including April.