KOSPI slips below 7,000 as US yields hit 19‑year high

South Korea’s benchmark index fell back under the psychologically important 7,000 mark after a rally‑filled morning was erased by a surge in U.S. Treasury yields that reached a 19‑year peak. The move matters because it shows that even a string of foreign inflows, a firmer won and upbeat semiconductor news cannot offset the global “rate‑shock” that is now shaping equity markets across the Pacific.

The Korea Exchange reported the KOSPI closed at 6,869.83 on 18 August, down 108.11 points or 1.55 %. The index had opened 2.15 % higher at 7,127.77 and climbed to a high of 7,216.62 before surrendering its gains. At one stage it slipped as low as 6,788.78, giving the session a trading range of 427.84 points — the widest in August.

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U.S. Long‑term yields were the chief drag. The 30‑year Treasury yield topped 5.32 %, a level not seen since 2007, while the 10‑year rate rose above 4.72 %. Analysts tied the yield jump to a convergence of a widening U.S. Fiscal deficit, heavier Treasury issuance and a surge in corporate debt from AI‑focused data‑center builders.

"Rising Treasury yields mean the liquidity environment is deteriorating,"

Lee Jin-woo, head of the research center at Meritz Securities, via Seoul Economic Daily

Even as foreign investors continued to add to Korean equities for a fifth straight session — netting 86.1 billion won on the 18th after a streak of 2 trillion‑to‑3 trillion won daily inflows from the 12th to the 14th, the higher yields eroded risk appetite. “The fact that Korean stocks have fallen a lot, making their undervaluation more attractive, along with positive news related to semiconductors and some easing of concerns about the exchange rate,” said Kim Tae‑hong, CEO of GrowthHill Asset Management.

The won had helped by easing to 1,411.8 per dollar, softening the currency burden for import‑heavy firms. Yet the same day saw Brent crude climb into the $91‑a‑barrel range and West Texas Intermediate near $85, reflecting renewed tension in the Middle East after a 60‑day U.S.–Iran ceasefire lapsed without progress.

"A steadier exchange rate is positive, the burden of rising oil prices and geopolitical risk continues,"

Lee Young‑gon, head of the research centre at Toss Securities, via Seoul Economic Daily

Across the region, the same bond pressure reverberated. Japan’s Nikkei 225 slid 2.54 % and Taiwan’s TAIEX fell 1.20 %. The Australian market likewise felt the pinch; the ASX falls as tech dropped after the U.S. 10‑year yield touched its highest level since early 2025 and the 30‑year hit levels last seen in 2007.

In the United States, a similar pattern unfolded. The U.S. Market saw futures point to a higher open as crude oil pulled back and the Treasury announced that it would double buyback operations for longer‑dated debt. Yet the backdrop remained a “long‑end” of the yield curve rattling equities, a trend echoed by Capital.com analyst Daniela Hathorn: “Instead, the long end is responding to persistent inflation risks, heavy government borrowing and growing competition for capital, including debt issuance associated with the AI investment boom.”

The AI‑related debt theme is also surfacing in the “neocloud” niche. A Wall Street Journal analysis highlighted roughly $3 trillion of off‑balance‑sheet AI commitments across nine major tech firms, dwarfing traditional capex. Companies such as Nebius Group and CoreWeave, which finance massive GPU build‑outs with debt, saw their shares tumble on Wednesday before partially recovering, despite a modest dip in the 30‑year yield to 5.21 % after the Treasury’s buy‑back announcement. Nebius Dives 14% reported that “the big driver of yesterday’s rotation out of technology and momentum names was rising interest rates.”

Key factors in the KOSPI slide

• U.S. 30‑year Treasury yield above 5.3 %, its highest since 2007.
• Foreign inflows continued but were outpaced by yield‑driven risk aversion.
• Oil price rise to the $91‑$92 Brent range amid renewed U.S.–Iran tension.
• Semiconductor optimism tempered by the looming impact of higher financing costs.
• Upcoming catalysts: Nvidia’s earnings report and the Jackson Hole symposium.

What to watch next

Event	Date	Potential impact
Nvidia earnings release	late August	Could boost or further dampen semiconductor‑linked equities.
Jackson Hole symposium	late August	Policy signals may clarify the trajectory of long‑term yields.
U.S. Treasury buy‑back expansion	effective 9 September	May ease the long‑end of the curve, supporting rate‑sensitive stocks.
U.S. Fed minutes (July meeting)	2 p.m. ET, 19 August	Will influence expectations for further rate moves.

Investors will be weighing whether the yield spike is a temporary market reaction or a sign that higher financing costs will linger, especially for capital‑intensive sectors such as AI infrastructure and semiconductors. The KOSPI’s failure to hold above 7,000 signals that, for now, the “rate‑shock” narrative dominates market sentiment.

The next few weeks will reveal whether the KOSPI can rebound on the back of stronger semiconductor earnings, or whether the higher‑yield environment will keep Asian equities on the defensive.