Seoul — August 18, 2026. Published after the market close; index and stock prices are official closing values, while the exchange rate is an indicative late-session level, as currency trading is continuous.
The 30-second version
- The KOSPI closed at 6,869.83, down 1.55% — after climbing as high as 7,216.62 in the morning. That is a swing of roughly 350 points (about 4.8 percentage points) from the intraday high to the close.
- The KOSDAQ fell harder, -3.52% to 834.20, as short-term money that had crowded into the semiconductor rally headed for the exits.
- Foreign investors were the only net buyers of KOSPI shares — +759.0 billion won (roughly $540 million) as of the last available intraday reading — while domestic individuals and institutions sold.
- Two forces drove the reversal: heavy profit-taking after a 2%-plus open, and the collapse of the US-Iran ceasefire at midnight on August 17.
- The won firmed slightly, with USD/KRW at around 1,412 per dollar, down about 0.2%.
From flirting with 7,200 to a 350-point slide
The day started as a continuation of the semiconductor euphoria. The KOSPI opened up 2.15% at 7,127.77 and pushed to 7,216.62, riding overnight strength in US chip stocks. SK Hynix jumped as much as +7.96% intraday and Samsung Electronics traded up 3.28% early on.
The spark came from the US: the Philadelphia Semiconductor Index rose, and Anthropic reported annualized revenue above $65 billion and its first-ever operating profit, along with a 2028 revenue forecast of $190-200 billion and talk of an IPO as early as this fall. For Korean investors, the read-through is straightforward — a bigger AI industry means more demand for the memory chips Korea makes — and that expectation powered the morning rally.
Then the afternoon happened. Individuals and institutions began locking in gains, program selling kicked in, and the index gave back the entire advance, finishing at 6,869.83. SK Hynix ended with a gain of only 1.03% at 1,662,000 won, and Samsung Electronics actually closed lower, down 2.19% at 268,500 won.
Foreigners bought; almost everyone else sold
One caveat up front: final confirmed daily flow data was not yet published at reporting time, so the figures below are from about 11:45 a.m., when the index stood at 6,894.59 — very close to the eventual close. The selling pressure most likely persisted into the final bell, but treat the exact numbers as near-final rather than official.
On the KOSPI, foreign investors were net buyers of 759.0 billion won (about $540 million at the day’s exchange rate) — and in the first hour of trading their buying had exceeded 1.06 trillion won (about $750 million). On the other side, individuals sold a net 428.4 billion won and institutions sold 272.3 billion won. Program trading — algorithm-driven basket trades that often track index arbitrage — flipped to a net sell of 232.8 billion won, which tends to accelerate a decline once it turns.
On the KOSDAQ, the pattern inverted: individuals bought a net 185.5 billion won while institutions (-161.9 billion won) and foreigners (-21.5 billion won) sold.
Market breadth told the real story of sentiment. At one point during the session, only 197 KOSPI stocks were rising against 696 falling, and on the KOSDAQ it was 400 up versus 1,268 down. A rally carried by a handful of chip names while the rest of the market bleeds is fragile, and today it broke.
The Middle East is back on the board
The 60-day US-Iran ceasefire expired at midnight on August 17 without a deal, reviving fears of a wider conflict. Repeated tanker attacks near the Strait of Hormuz are putting upward pressure on oil prices, and that reshuffled winners and losers in Seoul.
Shipping company HMM surged +7.06% — when sea lanes get riskier, freight rates tend to rise, which flows straight into shipping revenue. Hyundai Marine & Fire jumped over 10% to a 52-week high, though for a domestic reason: brokerages including KB Securities raised their price targets to 65,000 won on improving auto and health insurance profits. Chip-equipment names EO Technics (+4.08%) and Jusung Engineering (+2.81%) held on to gains.
The casualty list was broader: Samsung Electro-Mechanics fell 4.4-5.4%, LG Energy Solution dropped 4.2%, and Hyundai Motor lost 2.8-3.5%. The won strengthened modestly to around 1,412 per dollar (down about 0.2%), supported by foreign equity buying, though Middle East risk capped the move.
Key closing numbers
| Instrument | Close | Change | Note |
|---|---|---|---|
| KOSPI | 6,869.83 | -1.55% | Intraday high 7,216.62 |
| KOSDAQ | 834.20 | -3.52% | Intraday high 872.59 |
| USD/KRW | around 1,412 | down about 0.2% | Won slightly stronger; indicative level |
| SK Hynix | 1,662,000 won | +1.03% | Was up 7.96% intraday |
| HMM | — | +7.06% | Middle East / freight rates |
| Samsung Electronics | 268,500 won | -2.19% | Turned negative in the afternoon |
What to watch next
- FOMC minutes, August 19 (US time) — a hawkish tone could push Treasury yields and USD/KRW volatility higher.
- US-Iran escalation — the single biggest variable for oil prices, safe-haven flows, and shipping stocks after the ceasefire lapsed.
- Whether foreign buying persists — foreigners were the lone pillar under the KOSPI today; if they step back while locals keep selling, support thins quickly.
- Final confirmed flow data — today’s investor figures were near-close readings, so the official numbers may differ slightly.
FAQ
Why did the KOSPI fall on August 18 after hitting 7,216?
Profit-taking. After a 2.15% gap-up open driven by semiconductor optimism, individuals and institutions sold heavily in the afternoon, program trading flipped to net selling, and renewed Middle East risk gave everyone a reason to lock in gains. The index gave back the entire morning advance and more.
Did foreign investors sell Korean stocks today?
No — on the KOSPI they were the only net buyers, at roughly 759 billion won (~$540 million) as of the last intraday reading, after buying over 1 trillion won in the first hour. They were small net sellers of the KOSDAQ.
Why did the KOSDAQ fall so much more than the KOSPI?
Short-term money that had rotated into the semiconductor rally pulled out, and the smaller-cap KOSDAQ absorbed the outflow hardest. Breadth was dismal: at one point 1,268 KOSDAQ stocks were falling against just 400 rising.
What does Anthropic’s earnings news have to do with Korean stocks?
Anthropic disclosed annualized revenue above $65 billion and its first operating profit, reinforcing the AI capital-spending story. Korean chipmakers like SK Hynix and Samsung Electronics supply the memory that AI infrastructure runs on, so the news lifted them sharply at the open — before the afternoon selloff trimmed or erased those gains.
Disclaimer: This is an information and analysis publication, not investment advice. See our Methodology for data sources, standards, and our corrections policy.