The benchmark Korea Composite Stock Price Index lost 3.85 points, or 0.16 percent, to close at 2,412.2. Trade volume was moderate at 262 million shares worth 5.2 trillion won ($4.6 billion), with losers outnumbering gainers 593 to 216.
The local stock market opened a tad higher tracking overnight gains on Wall Street but lost ground as investors took to the sidelines ahead of the two-day Federal Open Market Committee meeting slated to finish Wednesday (local time), according to analysts.
Though the Fed is widely expected to leave interest rates unchanged, experts say the results could give clues to US monetary policy.
"A wait-and-see stance took hold among investors due to the FOMC meeting and the geopolitical tensions after US President Donald Trump's speech at the United Nations," said analyst Kim Byong-yeon at NH Investment & Securities.
In an address to the UN General Assembly in New York on Tuesday (local time), Trump said the United States will "have no choice but to totally destroy North Korea" if the reclusive regime continues with its nuclear threats.
Foreign investors offloaded a net 188 billion won of local stocks, while individuals and institutions snatched up a net 85 billion won and 58 billion won, respectively.
Market heavyweights had a mixed day of trading.
Tech shares closed higher, with top cap Samsung Electronics advancing 0.19 percent to close at 2,611,000 won. SK hynix, a major chipmaker, gained 1.89 percent to close at 80,700 won.
Carmakers also ended in positive terrain, with industry leader Hyundai Motor climbing 1.43 percent to close at 142,000 won and its sister firm Kia Motors rising 1.13 percent to 31,350 won.
Chemicals shares mostly lost ground, with market leader LG Chem losing 2.14 percent to close at 388,500 won and leading cosmetics company AmorePacific shedding 0.79 percent to 251,000 won.
Local retailers, which have been bearing the brunt of Beijing's apparent retaliation over Seoul's deployment of a US missile defense system, were also among the losers, with Lotte Shopping plunging 4.55 percent to close at 220,500 won.
Retail giant Lotte recently decided to withdraw its discount store chain from the Chinese market.
The local currency closed at 1,128.30 won against the US dollar, up 3.00 won from the previous session's close.
Bond prices, which move inversely to yields, ended higher. The yield on three-year Treasurys shed 0.4 basis point to 1.775 percent and the return on the benchmark five-year government bonds declined 0.6 basis point to 1.983 percent. (Yonhap)