The reserves totalled $295.96 billion at the end of last month, with an increase in deposits in foreign currencies offsetting a drop in securities, a table in the Bank of Korea statement showed.
It was the largest monthly gain since September 2010, when reserves expanded by $4.42 billion.
Most of the increase was attributed to the strengthening euro and British pound and thus a rise in their dollar-conversion value, it added, without providing further details.
The euro advanced about 2 percent against the dollar in January as a jump in inflation stoked expectations of interest rate rises.
Shin Jae-hyuck, a Bank of Korea economist, said the central bank had sold some of its securities holdings in reshuffling its reserves portfolios, while raising the amount of deposits.
South Korea's foreign reserves ($ billion, at end-month):