The linkage between South Korea's stock market and the U.S. technology share market is rising rapidly. Expanding investment in artificial intelligence (AI) infrastructure is moving earnings expectations for U.S. big tech as well as Samsung Electronics and SK Hynix in the same direction. That has pushed the correlation between the KOSPI and the Nasdaq 100 to its highest level since 2021.

CNBC reported on July 28 that the 60-day correlation between the KOSPI and the Nasdaq 100 has recently climbed to around 0.50.

Behind the shift is the larger weighting of Samsung Electronics and SK Hynix in the KOSPI. Their combined weighting accounts for more than half of the index. At the same time, the two companies sit at the core of the memory chip supply chain that goes into data centres run by large U.S. technology companies. Rolf Bulk (롤프 벌크), an analyst at Fuchs & Group, said the higher correlation comes as the KOSPI has effectively become closer to a semiconductor index.

A change in the demand structure for DRAM used in AI servers is amplifying that trend. Bulk pointed to the data-centre share of global DRAM demand rising from about 40 percent last year to more than half this year. He said he expects that share to grow further. Hyperscaler spending that supports U.S. semiconductor shares and big tech earnings affects Samsung Electronics and SK Hynix in the same way.

As a result, Asian investors are increasingly using the South Korean market ahead of the Wall Street open to gauge the intensity of global AI trades. Jung-in Yoon (윤정인), founder of Fibonacci Asset Management, said Samsung Electronics and SK Hynix show the first liquid market reaction to overnight issues that affect global AI demand. He said SK Hynix, in particular, has become an important barometer for demand for key AI supply-chain components because of its high exposure to high-bandwidth memory (HBM).

Recent trading patterns also show the trend. On July 13, the KOSPI fell more than 8 percent. SK Hynix plunged 15 percent, pulling the index lower. The Nasdaq 100 also closed down 1.88 percent the same day. Micron Technology fell 4 percent, while SanDisk dropped 12 percent and Intel slid 6 percent.

Some also say the market does not always treat one side as a leading indicator. Phillip Wool (필립 울), head of research at Ralliant Global Advisors, said a common factor moving U.S. and South Korean tech shares together is investor sentiment toward AI hardware trades. If AI-related news breaks when the U.S. market is closed, Samsung Electronics and SK Hynix can serve as proxy indicators of how Wall Street may react when it reopens, he said. Conversely, changes during U.S. trading hours can act as a signal for the next South Korean session.

Corporate earnings schedules also matter. Samsung Electronics typically releases earnings about 2 weeks earlier than major U.S. semiconductor companies. That means Samsung Electronics' earnings guidance can be read each quarter as a relatively early signal of the state of AI demand.

The deeper linkage also increases investment risk. The industry sees the regional diversification effect expected from holding U.S. and South Korean stocks together weakening. Bulk warned South Korea no longer provides a diversification tool against U.S. technology shares, and with half the index tied to a single cyclical theme, South Korea's market could be hit harder than others if hyperscalers slow capital spending. He added that South Korean memory chip shares could be more volatile than some U.S. semiconductor shares because they are influenced by leveraged exchange-traded fund flows.

Wool also said that if the two countries' tech shares are effectively driven by one large risk factor, the original purpose of overseas diversification weakens. The international diversification effect, which was a reason to invest in geographically distant markets such as the United States and South Korea, could shrink, he said.

Some also view the synchronisation as unlikely to become permanently entrenched. KB Financial Group's head of global investment strategy said Micron, Samsung Electronics and SK Hynix are all benefiting from rising DRAM prices for now, but share price trends could diverge later due to differences in capital spending, product mix and U.S. policies supporting domestic semiconductor production. Catch-up efforts by Chinese memory makers are also cited as a variable. He said Chinese firms still lag technologically but are advancing faster than investors expected. ChangXin Memory Technologies (CXMT) shares surged 466 percent on their first day of trading on Shanghai's STAR Market on July 27.