The KOSPI closed at 7,080.92 on September 23, 2026, up 63.01 points or 0.90% from the prior session. But that single number captures only half the story. The index opened by surging to 7,153.99, nearly a 2% jump in one day—then gave back most of those gains by late morning. Non-semiconductor sectors simply collapsed, leaving less than half the morning's advance intact.
What made today intriguing were the worst performers. Korea Electric Power, Doosan Enerbility, Hyundai Engineering and Construction, Daewoo Engineering and Construction—all tied to nuclear and infrastructure projects with the U.S. The morning brought word of expanded U.S. investment. Good news, in theory. Yet the market went the opposite direction.
Why did a 2% rally shrink to 0.9%?
The KOSPI started the day riding semiconductor strength from overnight, when the Philadelphia Semiconductor Index had jumped 2.06%. The index opened at 7,153.99, riding that momentum. But by late morning it had retreated to 7,014.98, nearly breaking even. The swing from high to low: 139 points, or almost 2 percentage points in a single session.
KOSPI daily chart (Naver Finance; red = up, blue = down)
Two factors drove the morning volatility. One was a cautious mood ahead of the Chuseok holiday. The other was a wave of disappointment selling from non-semiconductor sectors. The index later regained ground but never fully recovered. Still, the gain marked a fourth consecutive day of advances.
Investor flows tell the story. Institutional buyers returned to net buying after a one-day pause, while other corporates logged their 14th consecutive day of net purchases. Foreign investors, by contrast, flipped to net selling after four days of buying. Retail investors continued to sell for a fourth day straight. The four-day pattern of buyers versus sellers completely reversed today.
The KOSDAQ painted a simpler picture: it closed at 844.48, up 1.21%—a rebound after recent declines. Retail investors bought a net 30.6 billion won, institutional buyers 7.2 billion. Foreign investors sold 46.5 billion. The opposite of the KOSPI. This is a market where retail has been a net buyer for four straight days.
Compiled and created directly
KOSDAQ daily chart (Naver Finance; red = up, blue = down)
Semiconductors again—but with a fresh reason
Samsung Electronics rose 3.25%, SK Hynix 1.20%, Hanmi Semiconductor 1.46%. This much follows the pattern of recent days. The difference is the reason. Beyond memory-price strength, a new catalyst emerged: Meta's new AI agent Muse reached the No. 1 spot on Apple's free app chart. Optimism about AI infrastructure demand has sharpened. Overnight in New York, SanDisk jumped 6.82%. Micron rose 5.00%, and SK Hynix ADR gained 3.45%.
Samsung Electronics recent daily chart (Naver Finance; red = up, blue = down)
On the KOSDAQ, semiconductor materials and equipment stocks moved harder. PSK climbed 10.62%, Padu 12.86%, Ujin Tech 9.50%. Buying pressure concentrated on materials and equipment more than the broader semiconductor index. That was today's story.
Post-quantum cryptography names also surged. UriNet hit the daily limit at +29.90%; ExGate rose 10.16%. The catalyst was IonQ, which announced a breakthrough in solving a key technical hurdle to commercializing quantum computers. UriNet closed at the daily limit.
UriNet recent daily chart (Naver Finance; red = up, blue = down)
U.S. Investment Announcement Dragged Down Related Stocks
The worst-hit sectors today were nuclear power and construction. Doosan Enerbility fell 5.55%, LS ELECTRIC 3.07%, Korea Electric Power 1.94%. Construction stocks took a harder hit: Hyundai Engineering and Construction down 7.77%, Daewoo Engineering and Construction 7.23%. Sharp declines.
Hyundai Engineering and Construction recent daily chart (Naver Finance; red = up, blue = down)
The irony: news broke of a Westinghouse stake acquisition and expanded U.S. investment. But the market had expected a larger commitment. Disappointment spread that the announced size fell short of expectations. U.S. investment is normally bullish, yet today's weakness in related names shows that when the dollar figures missed hopes, it became bearish instead. The direction of the news was right, but the magnitude was wrong. The numbers were the problem.
On the same logic, shipbuilders (HD Hyundai Heavy Industries -3.18%, Hanwha Ocean -2.45%), steelmakers, LNG, and the broader theme of U.S. investment tied to Korea-North Korea-Russia gas pipelines all weakened together. Automakers (Hyundai Motor -1.94%, Kia -2.62%) and bank stocks (Shinhan Financial Group -3.45%, KB Financial -1.42%) also lost ground. It was a broad sector selloff.
On the flip side, some sharp gainers stood out. HT Robotics closed at the daily limit on optimism that LG Energy Solution's backlog of battery energy storage systems for North America could exceed 1 trillion won. KSS Shipping rose 17.89% after announcing a near-200-billion-won share buyback. Wiseplanet Company, which debuted on the KOSDAQ today, rocketed nearly 4 times its IPO price of 12,000 won to close at 46,300 won. A blistering first day.
What to watch next
Markets are closely tracking direct U.S.-Iran diplomatic talks and a planned U.S.-China summit. President Trump described a three-hour meeting between his Middle East envoy and Iran's foreign minister at the UN General Assembly as "very productive," and hinted at further talks soon. If this channel solidifies, shipping and defense names could re-emerge as movers.
Domestically, the next key is any follow-up announcement on Westinghouse. Because selling came in today on disappointment, a fresh disclosure of investment size or timeline could reverse direction for nuclear and construction stocks. The holiday calendar also tightens the window before Chuseok, another variable. Next announcement will set the tone.
Days when foreign investors sold in large volume were documented in earlier posts. KOSPI 7,007 Breach; Retail Dumped 3 Trillion but Institutions Bought for Four Days Straight and KOSPI 6,909 Crash; Foreign Investors Sold 2 Trillion as Insurance and Shipbuilder Names Rose Alone show how investor flows mapped to index moves.
This article is for informational purposes and does not recommend buying or selling any specific holding. Investment decisions and their outcomes are your own responsibility.
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