Why South Korea's KOSPI Plunged 7.57% in an AI-Driven Chip Stock Sell-Off

What happened on October 15

South Korea's benchmark KOSPI index dropped 454.59 points, or 7.57%, to close at 5,554.22 on October 15, 2026, according to the Korea Herald. It was the index's seventh consecutive losing session and its first close in the 5,500 range since July 30, with the index briefly touching an intraday low of 5,450.81 after opening down 5.59%.

The decline was fast enough to trip South Korea's automatic trading safeguards twice. A 'sell-sidecar' was activated at 9:07 a.m. local time, briefly suspending sell orders tied to index-futures program trading. A full market-wide circuit breaker followed at 12:38 p.m., halting trading on the exchange for 20 minutes, the Korea Herald reported. The tech-heavy KOSDAQ index fell 7.65% to 760.82 and triggered its own sidecar and circuit breaker; the Korea Herald reported this was the first time since July 29 that both exchanges activated these trading halts on the same day.

Which stocks fell the most

Semiconductor shares led the rout. Samsung Electronics and SK hynix are the world's two largest memory-chip makers and major suppliers to the data-center buildout behind generative AI, so their share moves are closely watched as a gauge of sentiment toward the broader AI trade, the Korea Herald and Korea JoongAng Daily reported.

Why investors sold: three forces cited by analysts

Korean financial press pointed to a combination of factors rather than a single cause. First, U.S. consumer price data released that week showed prices rising 0.6% month-on-month in September, the fastest monthly pace since April, according to the Korea Herald and the Korea JoongAng Daily. That raised concern the Federal Reserve could keep interest rates higher for longer, which tends to weigh on richly valued technology stocks.

Second, doubts resurfaced globally about whether the enormous capital being poured into AI infrastructure will generate matching returns. The Korea JoongAng Daily reported that AI-cloud company CoreWeave's shares fell 15.5% around the same period, with its corporate bond yields rising to roughly 15%, and that one analyst flagged slower third-quarter revenue growth at AI company Anthropic as a factor feeding the doubts.

Third, the Korea JoongAng Daily reported that worries about expanded memory-chip supply from China's CXMT, a Chinese memory-chip maker, compounded doubts about the profitability of AI investment. That report did not detail CXMT's technology or production timeline, and the other two sources reviewed did not mention CXMT.

Who was selling, and what it means beyond Korea

Foreign investors were the main sellers, offloading a net 3.62 trillion won of Korean shares on October 15 alone, according to the Korea Herald and UPI. The Korea JoongAng Daily reported this extended a streak of eight consecutive sessions of net foreign selling. Analysts cited by the Korea JoongAng Daily said a reversal would likely require U.S. long-term interest rates to fall before foreign capital returns to Seoul-listed shares.

Because Samsung Electronics and SK hynix supply memory chips used in AI servers worldwide, their sharp declines were read by Korean financial media as part of a wider reassessment of AI-related valuations rather than a problem isolated to Korea. The sell-off coincided with the CoreWeave and Anthropic concerns noted above, linking the Seoul rout to the same global AI-spending debate affecting U.S. tech and chip stocks.

None of the reports reviewed describe a resolution to the underlying question of whether AI infrastructure spending will prove profitable; analysts quoted by Korean outlets characterized it as an open concern driving volatility rather than a settled conclusion.