The preliminary figure compared with a revised surplus of $7.2 billion in August, the central Bank of Korea said.
The current account -- the broadest measure of foreign trade in goods and services -- has been in the black for 31 months in a row.
Exports last month stood at $51 billion, compared to $48.9 billion a month ago and $50.6 a year ago, thanks to increased shipments of memory chips and steel, the BOK said.
Imports rose to $43.3 billion, up from $41.6 billion a month ago and $42 billion a year ago.
The services account, which includes spending on overseas trips, reported a deficit of $280 million, compared to $730 million in August and $200 million a year ago.
The central bank forecasts South Korea -- Asia's fourth-largest economy -- will post a record current account surplus of $84 billion this year.