The KOSPI closed at 6,954.52, down 0.58 percent from the previous session.
The index opened above 7,000 and climbed steadily through much of the morning, reclaiming the level intraday for the first time in 15 trading sessions. It rose to an intraday high of 7,171.52 before reversing in the afternoon.
Selling intensified near the close and pushed the index as low as 6,951.78. It finished at 6,954.52, nearly 220 points below its intraday high.
The early rally was led by chip stocks as investors bet that OpenAI's new generative pre-trained transformer (GPT)-6 Astra model would boost memory demand.
Samsung Electronics rose earlier in the session before slipping 0.19 percent to 269,500 won at the close. SK hynix climbed as high as 1,889,000 won, up nearly 6 percent intraday, before giving back most of its gains to finish 0.56 percent higher at 1,793,000 won. SK Square rose 0.53 percent to 1,131,000 won.
Nuclear, power-equipment and construction shares also rallied on expectations that South Korean companies could participate in major U.S. power projects under Seoul's US$350 billion investment commitment. Reports have pointed to discussions over as many as eight large nuclear reactors and a gas-fired power project in Texas.
Doosan Enerbility jumped as much as 6.37 percent to 93,500 won on hopes for those projects before paring gains to close 1.82 percent higher at 89,500 won. Buying remained strong in other nuclear and construction shares at the close. Daewoo E&C rose 8.47 percent to 19,970 won, while KEPCO E&C surged 16.60 percent to 142,600 won.
The market lost momentum later in the day as international oil prices climbed further after an attack on a Saudi Aramco refinery. West Texas Intermediate futures rose above $93 a barrel, while Brent crude topped $97. Investors also turned cautious ahead of U.S. inflation data, with the producer price index due Thursday and the consumer price index on Friday.
Despite the late selloff, foreign and institutional investors remained net buyers. Foreign investors bought a net 631.6 billion won ($470 million) and institutions purchased 642.7 billion won, while retail investors sold 3.03 trillion won.
Electrical equipment fell 3.33 percent, IT services dropped 3.76 percent and electronic products lost 4.17 percent. Electronic equipment and devices were the weakest group, sliding 4.92 percent. Auto parts, air freight and logistics, and electrical products each fell about 2.9 percent.
Losses were broad among other large-cap stocks. Samsung Electro-Mechanics tumbled 5.78 percent to 1,370,000 won. LG Energy Solution fell 3.86 percent to 348,500 won. Hyundai Motor dropped 2.04 percent to 385,000 won.
Samsung Biologics declined 1.77 percent to 1,440,000 won, KB Financial fell 1.25 percent to 173,700 won and Samsung Life lost 1.28 percent to 308,000 won. Kia dropped 2.34 percent to 125,400 won, Hyundai Mobis slid 3.94 percent to 415,000 won and LG Electronics fell 4.21 percent to 205,000 won.
The junior KOSDAQ followed a similar intraday pattern, reversing an early gain to close 1.25 percent lower at 811.88. It climbed as high as 830.21 in morning trading before turning lower in the afternoon.
Institutions sold a net 217.9 billion won, while retail investors bought 149.1 billion won and foreign investors purchased 60.8 billion won.
Losses were broad across the market. General services, pharmaceuticals and textiles and apparel stocks each fell more than 1 percent. Machinery and equipment, chemicals and manufacturing stocks also lost more than 1 percent. Metals and entertainment and culture shares ended slightly higher.
Nuclear-related shares bucked the broader KOSDAQ decline. Orbitech, which provides nuclear services and inspection, hit the daily ceiling and closed 29.83 percent higher at 6,050 won. Woori Technology, which supplies control and monitoring systems for nuclear power plants, gained 8.15 percent to 11,810 won.
Among semiconductor equipment and component shares, Jusung Engineering, which makes semiconductor deposition equipment rose 3.95 percent to 194,600 won. Simmtech, a maker of semiconductor package substrates gained 1.67 percent to 121,800 won. FADU, a fabless chipmaker specializing in SSD controllers and storage solutions, fell 1.78 percent to 104,800 won.
Other chip equipment and component makers also finished lower. Wonik IPS fell 1.53 percent to 115,800 won. EO Technics dropped 2.08 percent to 446,500 won, while ISC declined 1.61 percent to 183,000 won.
Losses were steeper in biotechnology and robotics shares. Alteogen, a biopharmaceutical company known for its drug-delivery platform technology, fell 3.15 percent to 277,000 won. HLB, which develops anticancer drugs, dropped 3.95 percent to 31,650 won, while Rainbow Robotics, a developer of collaborative and humanoid robot platforms, slid 3.35 percent to 433,000 won.
In the currency market, the Korean won weakened to 1,345.2 per dollar from 1,340.5 a day earlier.
Across the region, Japan's Nikkei 225 fell 1.70 percent to 65,269.33 as a stronger yen weighed on exporters. China's Shanghai Composite rose 0.20 percent to 3,940.55, supported by stronger August exports, while Hong Kong's Hang Seng Index slipped 0.38 percent to 25,317.43 amid caution over rising oil prices and Middle East tensions.
With U.S. inflation data due later this week, investors are likely to remain sensitive to energy prices and any signals that could alter expectations for the Federal Reserve's interest-rate path.
AJP Takeaways
- South Korea's KOSPI closed at 6,954.52 on September 8, 2026, down 0.58 percent, after climbing as high as 7,171.52 earlier in the session before surging oil prices and caution ahead of U.S. inflation data erased the day's gains.
- Semiconductor shares and U.S.-linked nuclear and power stocks led the early rally, with SK hynix reaching 1,889,000 won intraday and KEPCO Engineering & Construction closing 16.60 percent higher at 142,600 won.
- Foreign investors bought a net 631.6 billion won ($470 million) of KOSPI shares and institutions purchased 642.7 billion won, while retail investors sold 3.03 trillion won as the market reversed sharply late in the session.
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