The index swung between an intraday low of 8,900.7 and a high of 9,253.0 before settling higher. Retail investors were net buyers of 2.12 trillion won, while institutions bought 330.6 billion won. Foreigners sold 2.54 trillion won.
SK hynix jumped 5.6 percent to close at 2,919,000 won, lifting its market capitalization to 2,080 trillion won. That edged past Samsung Electronics' 2,067 trillion won on a common-share basis, ending Samsung's uninterrupted lead since November 2000.
Samsung's preferred shares, worth roughly 179 trillion won, mean the broader Samsung Electronics capitalization remains larger. But on the common-share measure used for index weighting and benchmark positioning, SK Hynix moved ahead.
Samsung Electronics slipped 0.14 percent to 353,500 won. The split reflected investors' growing preference for SK hynix, whose lead in high-bandwidth memory chips for AI servers has reshaped the market's view of Korea's two biggest chipmakers.
Japan's Nikkei 225 rose 1.6 percent to close at a record 72,353, lifted by the same AI semiconductor rally that drove gains in Seoul, as easing oil prices on progress in U.S.-Iran talks added to the region's relief.
The Kospi opened lower and briefly fell below 8,901 before reversing in the afternoon. Sentiment improved as progress in U.S.-Iran nuclear talks eased oil-market pressure. Qatar and Pakistan said the two sides had agreed on a roadmap toward a final deal within 60 days.
Oil prices fell on the news, with Brent down 1.8 percent at $79.1 a barrel and WTI off 2.5 percent at $75.4. Lower oil prices eased concerns over imported inflation, reducing pressure on the Bank of Korea to hold rates higher for longer.
LG Electronics was another standout, surging 7.6 percent to 227,500 won after reports that senior LG Group executives had traveled to Nvidia's headquarters in Santa Clara to discuss cooperation in AI and robotics.
SK Square, whose value is closely tied to SK Hynix, rose 10.7 percent to 1,970,000 won. Among smaller semiconductor names, Aittec jumped 20.6 percent and Jeju Semiconductor gained 17.2 percent.
Auto shares fell. Hyundai Motor dropped 5.2 percent to 581,000 won and Hyundai Mobis lost 7 percent to 569,000 won as investors weighed a government review of South Korea's revised trade union law.
The Ministry of Employment and Labor said companies could be required to bargain with unions representing subcontracted workers if their working conditions are effectively controlled by the parent company. The clarification added pressure on Hyundai-related shares.
The Kosdaq also closed higher, adding 1.8 points, or 0.19 percent, to 968.4. The index moved between an intraday low of 944.3 and a high of 979.6. Retail investors sold 463.2 billion won, while foreigners bought 311.5 billion won and institutions added 150.2 billion won.
Yujin Tech surged 118 percent. Fadu rose 14.3 percent to 115,900 won and PSK gained 9.2 percent to 180,200 won, extending gains among smaller semiconductor names. Rainbow Robotics fell 2.9 percent to 573,000 won and Samchundang Pharmaceutical dropped 4.7 percent to 241,500 won.
In China, the Shanghai Composite gained 1.7 percent to 4,159, while Hong Kong's Hang Seng slipped 0.5 percent to 23,815.
The dollar held near 1,537 won, up 0.3 percent on the day.
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