The benchmark KOSPI closed at 7,017.91, up 0.15 percent from the previous session.
The index opened 2.20 percent higher at 7,161.61 and climbed as high as 7,171.44, up 2.34 percent, before steadily losing ground. It briefly turned lower in afternoon trading before recovering above 7,000.
The decline came as investors took profits ahead of the long Chuseok holiday weekend, while oil prices rebounded during Asian trading after falling sharply overnight.
Retail investors sold a net 1.61 trillion won (US$1.19 billion) of KOSPI shares. Foreign investors bought a net 76.2 billion won, while institutions sold 121.8 billion won.
Samsung Electronics, which closed the regular session 0.91 percent higher at 276,500 won, was trading at 275,000 won as of around 4:30 p.m. in after-hours trading, up 0.36 percent from the previous day's close.
SK hynix reversed its early rally and closed the regular session 1.50 percent lower at 1,840,000 won, down 1.77 percent at 1,835,000 won.
Other technology shares remained mostly higher in after-hours trading. Samsung Electro-Mechanics rose 3.64 percent to 1,481,000 won, while Samsung Electronics preferred shares gained 1.20 percent to 210,500 won. SK Square added 0.27 percent to 1,130,000 won.
LG Electronics climbed 6.17 percent to 215,000 won, while Daeduck Electronics jumped 9.72 percent to 111,800 won.
The junior KOSDAQ closed 0.23 percent lower at 834.38, ending a five-session winning streak.
Retail investors bought a net 156.3 billion won, while foreign investors sold 57.9 billion won and institutions sold 94.4 billion won.
As of around 4:40 p.m. in after-hours trading, biotech company Alteogen fell 2.35 percent to 249,000 won. The company develops biotechnology platforms including technology that can convert intravenous drugs into subcutaneous formulations.
EcoPro, the holding company of a battery materials group, rose 0.13 percent to 80,000 won. Cathode-materials maker EcoPro BM gained 0.48 percent to 103,900 won.
Semiconductor and display equipment maker Jusung Engineering fell 1.46 percent to 202,500 won. Wonik IPS, which makes semiconductor processing equipment, rose 0.23 percent to 129,300 won.
Rainbow Robotics, which develops humanoid, collaborative and mobile robots, was unchanged at 439,500 won.
EO Technics, which makes laser equipment used in semiconductor, display and printed circuit board production, fell 2.07 percent to 474,000 won.
Leeno Industrial, a maker of semiconductor test sockets and probes, jumped 7.31 percent to 74,900 won.
The Korean won strengthened sharply, trading at 1,361.2 per dollar as of 3:30 p.m.
Across the region, Chinese and Hong Kong stocks edged higher on gains in AI-related shares and optimism over U.S.-China talks. The Shanghai Composite rose 0.06 percent to 3,952.13, while Hong Kong's Hang Seng Index gained 0.05 percent to 25,055.29.
Japanese markets were closed for a national holiday between Respect for the Aged Day and the Autumnal Equinox, with trading set to resume on Thursday.
AJP Takeaways
- The KOSPI closed 0.15 percent higher at 7,017.91 after an early rally of more than 2 percent faded as oil prices rebounded during Asian trading.
- Samsung Electronics remained higher in after-hours trading, while SK hynix reversed its early advance and traded below its regular-session close.
- The KOSDAQ fell 0.23 percent to 834.38, while Chinese and Hong Kong stocks edged higher and Japanese markets remained closed for a national holiday.
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