The Korea Composite Stock Price Index climbed 1.7 percent on the first trading day of 2013. The KOSPI ended at 2,031.1 points, reaching a nine-month high.
The previous record was on April 3 of 2012, when it reached 2,049.2 points. Investor sentiment was buoyed by the U.S. Senate passing a bill to head off the "fiscal cliff."
Samsung Electronics shares surged 3.6 percent to a record W1.58 million (US$1=W1,064) leading the day's gains.
The won closed up W7.1 at W1,063.5 against the U.S. dollar, the strongest since W1,063 on Sept. 2, 2011. Forex authorities voiced worries that the won will continue to strengthen, making Korean exports more expensive overseas.
"We are concerned that the foreign exchange rate will head up with an influx of capital as the fiscal cliff is resolved," said Strategy and Finance Minister Bahk Jae-wan. He promised "active and phased response measures," hinting at massive intervention, but the remarks failed to reverse the won's strengthening trend on Wednesday.