The benchmark Korea Composite Stock Price Index shed 1.87 points, or 0.08 percent, to 2,140.33 in the first 15 minutes of trading.
After holding a two-day policy meeting, the Fed left interest rates unchanged and said it would begin reducing its bond portfolio starting next month.
Most large caps on the Seoul bourse traded mixed, with market kingpin Samsung Electronics gaining 1.07 percent.
SK hynix, a major chipmaker, was also up 0.37 percent.
Top automaker Hyundai Motor shed 1.41 percent, and its smaller affiliate Kia Motors fell 1.91 percent.
The local currency was trading at 1,133.50 won against the US dollar, down 5.20 won from the previous session's close. (Yonhap)