"The current account registered a surplus of 8.64 billion dollars, the largest monthly surplus ever," the Bank of Korea said on its website.
The current account is the broadest measure of the country's trade with other nations. South Korea is Asia's fourth-largest economy, after China, Japan and India.
May's total surplus was up sharply from April's $3.93 billion.
The bank's data showed May's goods account registered a surplus of $7.27 billion, a jump from a surplus of $3.54 billion in April. May's services account surplus, however, dipped to $1.13 billion from $1.45 billion in April.
Speaking to reporters, Kim Young-bae, head of the bank's economic statistics division, said falling oil costs slowed demand for imports in May, Yonhap News Agency reported. He said next month's surplus would be smaller than in May but added the surplus trend was expected to continue.
South Korean exports have been aided by the won, which, like other currencies, has weakened against the U.S. dollar on expectations the U.S. Federal Reserve would taper off its monetary easing sooner than expected.