KOSPI Crashes 10% in a Day: Is the AI Rally Starting to Crack?

For most of this year, South Korea looked like the biggest winner of the AI boom. Its benchmark, the KOSPI, was up more than 90% year-to-date and had roughly tripled over the past year, repeatedly hitting record highs as investors poured money into semiconductor giants like Samsung Electronics and SK Hynix. AI enthusiasm was so strong that many investors seemed willing to pay almost any price for companies connected to the theme.

Then, in a single trading session, reality struck.

The KOSPI plunged 9.99% — just shy of 10% — triggering a market-wide circuit breaker and forcing the Korea Exchange to halt trading for 20 minutes. By the close, the index had finished at 8,203.84, down 910.71 points, wiping out a large chunk of the recent euphoria.

And the shock wasn't limited to South Korea. The selloff rippled across Asia. Japan's Nikkei fell, and Indian markets came under pressure too. The Sensex dropped more than 200 points, with IT stocks such as Infosys and TCS among the laggards as the Nifty IT index slid over 2% on the global technology selloff.

The obvious question is, what changed so suddenly?

A Rally That Got Ahead of Itself

To understand the fall, you first have to understand the rally.

The KOSPI delivered extraordinary returns before this correction. Over the past year the index had surged roughly 200%, and its year-to-date gain still stood above 90%. Those numbers are rare for any major stock market.

The problem with gains that fast is that expectations start rising even quicker than prices. Investors stop asking whether growth is strong and start assuming it will stay strong indefinitely. That creates a fragile setup, because once expectations get too high, even a small disappointment can trigger a large correction.

The trigger this time was an overnight selloff in U.S. technology stocks, combined with renewed worries about U.S. interest rates. Analysts described the move as profit-taking in semiconductors after a historic run, rather than a sign of any deterioration in the underlying businesses.

The Whole Market Was Riding on Two Stocks

One of the biggest risks in the KOSPI rally was concentration.

Samsung Electronics and SK Hynix, which are the world's largest and second-largest memory chipmakers, weren't just participating in the rally. They were driving it. Together, the two companies account for nearly half of the KOSPI's market value (about 48%) and have contributed roughly 70% of the index's 2026 gains.

Think about that for a moment. Close to half the benchmark's weight sits in just two companies. As long as both stocks keep rising, the index looks healthy. But when they fall, the entire market feels it.

And that's exactly what happened. Samsung Electronics fell about 12.3%, while SK Hynix dropped about 12.5% during the session.

The decline was sharp because these stocks had become symbols of the global AI trade. Investors weren't just buying semiconductor companies anymore; they were buying the future of artificial intelligence. When expectations get that ambitious, the corrections tend to be just as dramatic.

The Warning Signs Were Already There

The selloff may have looked sudden, but the warning signs had been building.

SK Hynix had climbed for several straight sessions, pushing its 2026 gain to nearly 190%. At that point, many investors began wondering whether prices were running ahead of reality, not because the companies were performing poorly, but because the market had already priced in enormous future success.

The higher a stock climbs, the harder it becomes to justify further gains.

Foreign Investors Headed for the Exit

Another factor that deepened the fall was foreign selling.

Foreign investors offloaded a net 5.79 trillion won (about $3.8 billion) of KOSPI shares during the session. When large institutional investors start reducing exposure at that scale, markets can move fast.

Retail investors reacted very differently. Instead of selling, they stepped in aggressively, buying a record net 11.11 trillion won worth of shares, a sign that many local investors still believe the AI story is intact. That confidence is about to face an important test.

Why Everyone Is Watching Micron

The next big test for South Korean stocks won't happen in Seoul.

It comes on June 24, when American memory-chip maker Micron Technology reports its quarterly earnings. Investors are treating Micron's results as a report card for the entire memory and AI semiconductor industry.

If demand for memory chips stays strong, confidence in the AI-driven chip story could return quickly. But if the numbers disappoint, investors may start questioning whether valuations have run too far ahead of fundamentals. In many ways, the KOSPI's next move may depend less on what happened this week and more on what Micron reveals next.

This Was Bigger Than South Korea

The selloff wasn't a purely Korean story. Japan's Nikkei 225 fell about 1%, and weakness spread across other Asian markets as investors trimmed exposure to technology and AI-linked stocks.

The timing matters. For months, AI has been the dominant investment theme globally, and markets rewarded almost anything tied to semiconductors, advanced computing, and artificial intelligence. Now investors are turning more selective. The conversation is shifting from "how big can AI become?" to "how much of that future growth is already priced in?" That's a very different question, and it often marks the point where markets get more volatile.

What It Means for Indian Investors

India isn't directly exposed to Korean chipmakers, but it is exposed to global AI sentiment. When a market like the KOSPI cracks, risk appetite cools everywhere, and India's IT and tech-linked names tend to feel it first, which is why Infosys and TCS led the Sensex lower on the same day.

The bigger takeaway is about concentration and expectations. A handful of stocks carrying an entire index is a risk Indian investors should recognise too, given how much of the Nifty's moves can hinge on a few heavyweights. The practical lesson is the one that never changes: diversify, size your positions sensibly, and don't assume that a stock going straight up will keep doing so. If you trade these swings actively, managing risk matters more than chasing momentum, our guide to intraday trading strategies walks through how to do that. You can also track these markets in real time on Sahi.