The December total was up $1.45 billion from a month earlier. The reserves included securities, deposits, gold bullion, Special Drawing Rights and the country's International Monetary Fund reserve position, the Bank of Korea said.
Yonhap News quoted BOK as saying a weaker U.S. dollar boosted the dollar-conversion value of non-dollar assets, while investment profits also rose.
Korea is the fourth-largest economy in Asia after China, Japan and India.
As of the end of November, the latest month for which figures were available, South Korea's reserves were the seventh-largest in the world, behind China ($3.66 trillion), Japan ($1.26 trillion), Switzerland ($531.2 billion), Russia ($515.6 billion), Taiwan ($415.6 billion) and Brazil ($362 billion), the BOK data showed.