The latest forecast by the Bank of Korea would the slowest growth rate in three years. The bank said exports, the main stay of the country's economy, were expected to cool because of the euro zone debt crisis as well as the slowdown in China, Yonhap News Agency reported.
The bank also lowered its forecast for next year to 3.2 percent from its previous estimate of 3.8 percent.
In other actions, the central bank lowered the key interest rate to 2.75 percent from 3 percent, the second such reduction this year.
South Korea is Asia's fourth-largest economy after China, Japan and India.
Separately, the bank said it plans to keep inflation between 2.5 percent and 3.5 percent over the next three years.