The local currency was quoted at 1,116.0 against the dollar at the end of onshore trade, setting a fresh six-month high. The won had closed Friday's Seoul session at 1,117.2 against the dollar.
The won briefly touched a high of 1,113.3 due to demand stemming from foreigners' stock buying, but gradually pared its gains as dollar demand rebounded.
"Some carryover short-cover positions and steady importer dollar demand provided a firm floor for the dollar," a local bank dealer said.
Some dealers also said wariness about intervention from the local foreign-exchange authorities to slow the local currency's rise also provided strong support for the dollar.
The benchmark Korea Composite Stock Price Index was down 0.3 percent at 2,002.35. But foreigners were net buyers of 503.1 billion won ($450.38 million) worth of local stocks on Monday.
Local bonds fell as the Fed's monetary easing led to continued profit-taking by investors. September futures on three-year treasury bonds fell 0.1 points to 105.87.
Yields on the benchmark five-year treasury bonds and three-year treasury bonds each rose four basis points from Friday's close.