The won was quoted at 1,105.5 to the dollar at the end of onshore trade, firming from 1,107.2 at the end of onshore trade on Tuesday.
But the won retreated from its intraday high of 1,103.3, due in part to importers buying the dollar. Some dealers also said they were concerned about the potential for intervention by Korean authorities to slow the won's gains.
The benchmark Korea Composite Stock Price Index ended up 0.7 percent at 1,955.15 points. Foreigners were net buyers of 20.6 billion won worth of local shares on Wednesday.
Korean bonds fell, with foreigners dumping futures contracts based on technicals. December futures on three-year treasury bonds ended down 0.08 points at 106.16.
Yields on the benchmark five-year treasury bonds and three-year treasury bonds both rose 3 basis points each.