On July 31, 2026, South Korea's main stock market, the KOSPI, jumped 18% in a single day. That was the largest one-day gain in its history. The surge was led by two giant technology companies: SK Hynix, which rose 30% (the maximum allowed in one day), and Samsung Electronics, which closed 26.8% higher — also its biggest one-day jump ever (Chosun).

What set off the rally? Earnings reports from major US technology companies that renewed excitement about artificial intelligence. American semiconductor (computer chip) companies rallied, and that enthusiasm spread to South Korea (Seeking Alpha). Bloomberg pointed to the US rally and a share purchase by SK Hynix's chairman as key drivers (Bloomberg).

July was a wild month for both companies. SK Hynix started trading on the US Nasdaq exchange on July 11, with shares jumping more than 12%, before falling 7.9% to $154.70 two days later (Reuters). The company had launched a $28 billion US listing to tap into global demand for AI technology, having already outperformed Samsung and US-based Micron as one of the biggest winners of the AI boom (Reuters). On July 15, SK Hynix jumped nearly 13% in Seoul after US stocks rose on lower-than-expected inflation data (Reuters).

Then both companies reported their quarterly earnings. SK Hynix said its profit was up 557% from a year earlier, thanks to demand for AI chips — but that still fell short of what analysts had predicted, and the stock dropped about 10% on July 28 (Reuters). Samsung reported the next day: its profit was up 19-fold, with its chip division's profit rising more than 250-fold to $61.7 billion, making up for a loss in its phone division (Reuters).

Three trading days later, both stocks had their record rallies. The swing from a post-earnings sell-off to the biggest single-day gains ever was not caused by anything new happening in South Korea. It was driven by the US tech earnings cycle. CNBC reported on July 28 that US and Korean tech stocks are now tightly connected, because Samsung and SK Hynix depend on the same big spending by companies like Amazon, Microsoft, and Google on AI data centers that drives earnings for US tech firms (CNBC). Strong US earnings fed directly into Seoul's market open and produced historic gains.

Underneath the headlines, everyday investors were doing something striking. Individual investors sold a record 10.5 trillion won (about $7.6 billion) worth of KOSPI stocks on July 31 — the largest amount of retail selling ever recorded (Chosun). In other words, on the KOSPI's biggest-ever up day, regular investors were rushing for the exits, while big institutions and foreign buyers soaked up what they sold.

The broader context here is how much Korean chip stocks have detached from their own business results and attached themselves to the mood around US tech companies. SK Hynix grew profits 557% and that was treated as a letdown. Samsung's profit rose 19-fold and investors barely reacted at first. But US earnings results, which contained no new information about Korea, produced record gains for both. For anyone trying to understand these stocks, the message is that the US earnings calendar now matters more than Korean earnings reports. And the 10.5 trillion won in retail selling suggests local investors saw this rally as a chance to cash out, not a reason to stay invested.