South Korea's KOSPI index jumped 18% on July 31, 2026 — its biggest single-day gain ever — fueled by explosive rallies in semiconductor giants SK Hynix and Samsung Electronics. SK Hynix hit its 30% daily upper circuit (a trading limit that caps how far a stock can rise in one day), while Samsung closed 26.8% higher, also its largest one-day advance on record (Chosun).

The trigger was a wave of US tech earnings results that rekindled optimism around artificial intelligence. American semiconductor companies rallied in tandem, and that enthusiasm carried over to Seoul (Seeking Alpha). Bloomberg attributed the surge to the US peer rally and a share purchase by SK Hynix chairman Chey Tae-won (Bloomberg).

The move caps a volatile July for both companies. SK Hynix's US-listed shares debuted on Nasdaq on July 11, jumping more than 12%, before falling 7.9% to $154.70 two sessions later (Reuters). The company launched a $28 billion US listing in July to capitalize on global AI demand, having already outperformed Samsung and US-based Micron as one of the world's largest beneficiaries of the AI infrastructure buildout (Reuters). On July 15, SK Hynix jumped nearly 13% in Seoul, tracking US stock gains after softer-than-expected US inflation data (Reuters).

Then came earnings. SK Hynix reported Q2 2026 profit up 557% year-on-year on AI chip demand but missed analyst forecasts, sending shares down roughly 10% on July 28 (Reuters). Samsung, reporting a day later, posted Q2 profit up 19-fold: its chip division profit rose more than 250-fold to $61.7 billion, offsetting a swing to loss in its mobile division (Reuters).

Three sessions later, both stocks staged their record rallies. The reversal from the post-earnings selloff to all-time single-day gains was driven not by Korea-specific catalysts but by the US tech earnings cycle. CNBC reported on July 28 that US and Korean tech stocks are now tightly linked, with Samsung and SK Hynix increasingly dependent on the same hyperscaler capital expenditure — the massive spending on data centers and AI infrastructure by companies like Amazon, Microsoft, and Google — that drives earnings at US semiconductor and cloud companies (CNBC). That correlation was on full display: strong US tech earnings filtered into Seoul's open and produced historic moves.

Beneath the index-level euphoria, the trading flow tells a more ambivalent story. Individual investors sold a record 10.5 trillion won (roughly $7.6 billion) on the KOSPI on July 31, the largest retail selling on record (Chosun). In other words, the KOSPI's biggest-ever up day was met with retail investors exiting en masse, with institutional and foreign buyers absorbing the selling.

The broader context here is the degree to which Korean semiconductor equities have decoupled from their own fundamentals and re-coupled to US hyperscaler sentiment. SK Hynix's 557% profit jump was treated as a disappointment. Samsung's 19-fold earnings surge went initially unrewarded. Yet a US earnings print, with no new Korea-specific information, produced record gains in both names. For market participants, the practical implication is that entry points and risk management for KOSPI semiconductor exposure now require monitoring Nasdaq earnings calendars as a primary input, with Korean corporate results functioning as a secondary, almost lagging, signal. The 10.5 trillion won in retail selling suggests that local investors, at least, viewed the rally as a liquidity event to exit into rather than a fundamental re-rating to hold.