Foreign exchange reserves held by the central bank edged down by $1.58 billion in May to $370.90 billion after rising in two previous months, the central bank said in a statement.
The decline in value of some currencies such as the euro and Japanese yen against the US dollar led to the fall in reserves last month, the Bank of Korea said. The euro fell 1.8 percent in May while the yen slipped 2.6 percent, data from the central bank showed.
As of end-May, 90.4 percent of the central bank's foreign reserves were in the form of securities, including treasury and corporate bonds.
South Korea has the world's seventh-largest foreign reserves.