But dealers said further gains were capped by speculation of possible intervention by local foreign exchange authorities to prevent the currency of the export-reliant nation from rising too high.
The won stood at 1,090.7 against the dollar at the end of onshore trade, compared to Tuesday's close at 1,091.5. It firmed as high as 1,090.0, the highest since Sept. 9 last year.
"Most Asian currencies are on a strengthening track and the Korean won has developed a strong tolerance to euro zone events, giving little reason for the won to head down at this point," said a dealer in Seoul.
The same dealer expected the won to continue to inch up, despite efforts by local authorities to curb the won's rally.
The benchmark Korea Composite Stock Price Index was up 0.7 percent, with foreigners selling a net 37.0 billion won ($33.90 million) worth of South Korean stocks.
South Korean bond prices rose after data showed growth in September industrial activity was less than expected.
December futures on three-year treasury bonds were up 0.05 points to 106.25.
Yield on the benchmark five-year treasury bonds shed 2 basis points while the yield on the three-year treasury bonds fell 1 basis point.
South Korea is expected to post its first gain in exports in four months when it releases trade as well as inflation date for October on Thursday.