The benchmark KOSPI closed higher at 6,971.35, up 1.95 percent from the previous session. The index opened 0.34 percent lower at 6,814.49 and fell as low as 6,765.06 before reversing higher through the session.
Institutions bought a net 334.4 billion won ($246 million) of KOSPI shares, while retail investors sold about 1.43 trillion won and foreign investors unloaded 544.2 billion won.
The rebound followed Micron's fiscal fourth-quarter results, which beat market expectations and reinforced confidence in demand for memory chips used in artificial intelligence (AI) infrastructure.
Micron reported fiscal fourth-quarter revenue of $54.23 billion and adjusted earnings of $33.42 per share. The company forecast revenue of about $61.5 billion for the following quarter, also above market estimates. Customer financial commitments under its long-term supply agreements increased to about $32 billion from $22 billion in June as AI-related memory demand continued to expand.
The results helped semiconductor stocks recover after several sessions of pressure from high U.S. Treasury yields and foreign selling.
Samsung Electronics closed the regular session 2.79 percent higher at 276,000 won, while SK hynix gained 3.21 percent to 1,833,000 won.
Both stocks pared some of their regular-session gains after the close. In KRX after-market trading as of 4:34 p.m., Samsung Electronics was at 273,000 won, while SK hynix traded at 1,815,000 won.
Samsung Electronics preferred shares were up 3.23 percent at 201,500 won, while Samsung Electro-Mechanics gained 2.05 percent to 1,544,000 won. SK Square, the largest shareholder of SK hynix, was up 0.26 percent at 1,142,000 won.
South Korea's latest export data added to optimism over the chip cycle.
Semiconductor exports surged 262.8 percent from a year earlier to a record $60.3 billion in September, helping lift South Korea's overall exports to a record $120.94 billion, up 83.5 percent year on year, according to the Ministry of Trade, Industry and Resources (MOTIR) Thursday.
Energy and infrastructure stocks were another focus after South Korea and the United States unveiled plans for projects under Seoul's $200 billion strategic investment commitment.
The two countries agreed on a framework to use a $120 billion strategic investment fund to build eight large nuclear reactors in the United States, including two Korean-designed APR1400 units and six Westinghouse AP1000 units, with Korean companies expected to participate in the projects.
The plans lifted nuclear-related shares. Wonil T&I hit the daily limit, jumping 29.94 percent to 10,850 won, while KEPCO Engineering & Construction rose 2.61 percent to 125,800 won and Doosan Enerbility gained 0.61 percent to 82,100 won.
Steel shares also rose on expectations that the U.S. energy projects could boost demand for construction materials and pipelines. Daehan Steel surged 16.82 percent to 9,100 won, while SeAH Besteel Holdings gained 4.56 percent to 47,000 won and Dongkuk Steel advanced 4.55 percent to 9,880 won.
Investors also focused on the proposed Alaska LNG project, which includes a roughly 1,300-kilometer pipeline from Alaska's North Slope to an LNG export terminal. The project could create substantial demand for steel pipes if it proceeds.
Other major KOSPI stocks also remained higher in after-market trading.
LG Energy Solution was up 0.70 percent at 359,500 won, Hyundai Motor gained 0.72 percent to 347,500 won and Samsung Biologics rose 2.44 percent to 1,425,000 won.
The KOSDAQ posted an even stronger advance.
The junior index jumped 4.48 percent to close at 894.29. It had opened at 853.96 and briefly fallen to 851.92 before turning higher.
Foreign investors bought a net 230.8 billion won of KOSDAQ shares and institutions purchased 568.2 billion won, while retail investors sold about 776.1 billion won.
The advance broadened beyond large-cap memory stocks as stronger Micron capital-spending expectations supported semiconductor equipment makers, while biotechnology and battery stocks also attracted buying.
The strength continued into after-market trading.
Wonik IPS, a semiconductor equipment maker focused on deposition and other chip-manufacturing systems, was up 7.76 percent at 141,700 won as of 4:34 p.m.
EO Technics, which supplies laser-processing equipment for semiconductor, display and printed-circuit-board production, gained 6.54 percent to 554,000 won.
Jusung Engineering, a maker of semiconductor deposition equipment including ALD and CVD systems, rose 2.37 percent to 238,000 won.
LEENO Industrial, which makes semiconductor test probes and IC test sockets, climbed 9.95 percent to 80,700 won.
Simmtech, a supplier of semiconductor package substrates, gained 4.99 percent to 157,800 won.
HPSP, a semiconductor equipment company, rose 7.22 percent to 68,300 won.
Battery shares also remained firm. EcoPro, the holding company of a battery-materials group, rose 4.59 percent to 84,300 won, while cathode-material producer EcoPro BM advanced 4.03 percent to 113,500 won.
Alteogen, a biotechnology company developing platform technologies for biologic drugs, gained 3.34 percent to 263,000 won.
Rainbow Robotics, which develops collaborative, mobile and humanoid-based robot platforms, rose 3.59 percent to 433,000 won.
The turnaround also marked a sharp change from the previous session, when the KOSPI fell for a third straight day as foreign and institutional selling overwhelmed early gains in chip stocks.
Still, high U.S. borrowing costs remained a source of pressure. The U.S. 10-year Treasury yield had climbed above 5.3 percent overnight, keeping investors sensitive to the outlook for inflation and interest rates.
The Korean won weakened to around 1,362.70 per dollar from 1,361.6 earlier Thursday.
Across the region, Japan's Nikkei 225 surged 3.30 percent to 68,956.72 as chip and AI-related shares rallied on strong semiconductor demand.
Hong Kong and mainland Chinese markets were closed for the National Day holiday.
AJP Takeaways
- The KOSPI reversed an early decline to close 1.95 percent higher at 6,971.35 on October 1, supported by institutional buying and a rebound in semiconductor shares.
- Micron Technology's record fiscal fourth-quarter revenue and stronger-than-expected guidance lifted sentiment toward Samsung Electronics, SK hynix and South Korean semiconductor equipment makers.
- South Korea's September semiconductor exports surged 262.8 percent year on year to a record $60.3 billion, adding to expectations that AI-driven memory demand remains strong. Korea.kr
- The KOSDAQ jumped 4.48 percent to 894.29 as foreign and institutional investors bought semiconductor equipment, battery and biotechnology shares.
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