KOSPI jumps nearly 18%; SK Hynix (000660.KS) closes limit-up at 29.95%

KOSPI rose nearly 18% on July 31 and closed at 6,595.45, with Yonhap Infomax reporting the first one-day gain of more than 1,000 points in the index's history. SK Hynix (000660.KS) closed 29.95% higher at KRW 1,718,000, reaching South Korea's daily price limit and holding that level into the close.

The single-stock catalyst had a filing trail. Yonhap Infomax reported, citing the Financial Supervisory Service's DART system, that SK Group Chairman Chey Tae-won bought 3,620 SK Hynix shares on July 30 at an average price of KRW 1,353,677. The purchase totaled KRW 4.90031 billion. At the next day's closing price, those shares were worth KRW 6.21916 billion, implying an unrealized gain of about KRW 1.3 billion and a 27% return in one session.

Index-level flow was the second number. Maeil Business Newspaper reported that foreign investors bought more than KRW 7 trillion of KOSPI shares on the day, helping reverse a three-day selloff. CNBC also reported that SK Hynix gained almost 30%, Samsung Electronics rose nearly 27%, and the rally followed an 8%-plus overnight jump in the iShares Semiconductor ETF (SOXX) after stronger U.S. cloud and AI-spending signals.

This is not a routine closing recap because the index move exceeded the Korean Tier-1 threshold of ±3% by a wide margin and included a large-cap semiconductor limit-up move. The next checks are whether foreign net buying remains in cash equities, whether leverage-product rule changes reduce intraday volatility, and whether HBM and memory demand expectations translate into revised SK Hynix earnings forecasts.