On October 15, 2026, sell orders flooded the Korean stock market. The KOSPI closed at 5,554.22, down 454.59 points from the previous day, and the KOSDAQ ended at 760.82, down 63.06 points. Both indexes fell more than 7% in a single day.
During the session, measures to briefly halt trading were triggered on the KOSDAQ and then the KOSPI, one after the other.
✦ Oct 15 Close at a Glance
KRX regular session, Korea time
Explaining that day's drop with a single piece of news would miss the bigger picture. A sign that US prices were rising again, a selloff in chip stocks, and worries about the money and debt going into AI infrastructure all landed around the same time.
Let's separate the confirmed numbers from the market's interpretation.
The Numbers on October 15
✦ Intraday Moves on Oct 15
From open to close
The KOSPI opened at 5,673.01. It hit an intraday high of 5,751.45 and fell as low as 5,450.81 before finishing at 5,554.22. The close was 103.41 points above the intraday low, but even by the close it was still 7.57% lower than the previous day.
✦ KOSPI Closing Prices
Sep 28 to Oct 15, KRX regular session
The KOSDAQ opened at 784.45, rose to 788.69, slid to 745.43 intraday, and closed at 760.82. Not only the large caps of the KOSPI but also the KOSDAQ index, which has a bigger share of small and mid-sized stocks, fell sharply.
That the two indexes dropped by a similar amount shows the selling pressure wasn't confined to one side of the market.
✦ KOSDAQ Closing Prices
Sep 28 to Oct 15, KRX regular session
If you only look at one day's numbers, it's easy to miss how long the decline lasted. The KOSPI closed at 7,003.74 on October 2, then fell for seven straight trading days from October 6 through 15.
Compared with the October 2 close, the October 15 close was about 20.7% lower. Compared with the September 28 close of 6,889.74, it fell about 19.4%. The KOSDAQ also dropped about 17.3%, from 919.92 on October 6 to 760.82 on October 15.
It looks less like a one-off shock and more like a weakness that had been building for days and got sharper on the 15th.
US Inflation Shook Rate Expectations
✦ Four Things That Overlapped
Listed, not ranked
The first backdrop was the US consumer price index (CPI) for September. The US Bureau of Labor Statistics released the data at 8:30 in the morning local time on October 14. In Korea time, that's 9:30 in the evening on October 14.
It was information that came out before the Korean market opened on the 15th.
US CPI rose 0.6% from the previous month on a seasonally adjusted basis, and the year-over-year rate, not seasonally adjusted, was 3.7%. The gasoline index rose 9.1% in a month, accounting for more than half of the overall monthly increase in prices.
Core CPI, which excludes food and energy, rose 2.6% year over year. Energy prices pushed overall inflation up.
A single price indicator doesn't directly decide the US Federal Reserve's decision. But when prices rise again, expectations that rate cuts will be delayed or that high rates will last longer can gain strength.
That shift in expectations affects stock values too.
Investors calculate how much their future earnings are worth today, and when the discount rate used for that goes up, the present value of companies expected to make big profits several years from now can fall.
Still, you can't say that CPI alone explains the KOSPI's plunge. You also have to look at the selling already underway in the Korean market and the weakness in global tech stocks.
Chip Selling and Worries About AI Investment Costs
In Korea, big chip stocks like Samsung Electronics and SK hynix were pushed down hard. In the US, too, tech weakness continued, with the Philadelphia Semiconductor Index falling 4.3% the day before.
Looking at the two reports together, it was a decline tied to risk aversion across global chip and tech stocks rather than a problem unique to Korean companies.
As AI services grow, demand for high-performance chips and memory can rise. But investors don't look only at demand forecasts.
They also weigh the cost of expanding production capacity, how much prices and profit margins can hold up when competition intensifies, and how long it takes to recover the money spent in cash.
A falling stock price alone doesn't let you conclude that a company's earnings suddenly worsened that day, but it's worth considering that investors may have lowered their expectations for future earnings or tried to reduce risk.
The funding problems of AI infrastructure companies can be seen in the same context. AI compute servers and data centers require huge capital spending, and some companies have used borrowing to expand quickly.
In reports on October 15, the bonds of AI cloud company CoreWeave fell in price, and their yield in the secondary market was said to have topped 15%. When a bond's price falls, its yield usually rises.
It can be read as a sign that investors are demanding a higher reward for lending money.
However, a bond yield above 15% doesn't mean CoreWeave has already defaulted.
The price at which existing bonds trade in the market and the interest rate a company pays when it borrows anew are not the same, but a sharp rise in the yield on existing bonds can be a clue that investors' credit worries have grown.
AI demand growing and an AI infrastructure company generating enough cash flow to cover its investment costs are different problems.
Foreign Selling and the Market Safeguards
Foreign investors' selling was also mentioned in the market that day. That doesn't mean foreign selling was the only cause of the drop. But when sell orders pile onto large-cap chip companies, the whole index can be pushed down quickly.
The KOSPI is an index that combines many companies, but it is more affected by price changes in the big ones.
✦ Oct 14 to 15 in Order
Releases and market measures
The exchange's market stabilizers also kicked in one after another. First, sidecars were triggered on the KOSPI and the KOSDAQ, and afterward a Stage 1 circuit breaker went into effect in both markets.
According to Yonhap News, the KOSDAQ circuit breaker was triggered at around 12:23:13 Korea time, when the index was 757.88, down 8.01% from the previous day.
The KOSPI's trading was halted about 15 minutes later, near 5,528.05, down 8.00% from the previous day.
✦ Sidecar vs Circuit Breaker
What is different
A sidecar is a device that limits the validity of program trading orders for 5 minutes when futures prices swing beyond a set threshold. It doesn't stop all trading apart from program trading.
A circuit breaker, on the other hand, halts market trading for 20 minutes when set conditions are met, such as the index staying down 8% or more from the previous close for one minute. Even after trading resumes, single-price trading continues for 10 minutes.
Both give the market time to look at orders and prices again as things move fast, but neither is a device that puts stock prices back to where they were.
The PPI Came Out After the Korean Market Closed
The US producer price index (PPI) for September was released at 8:30 in the morning local time on October 15, which is 9:30 in the evening the same day in Korea time. That's six hours after the Korean regular session's closing time of 3:30 in the afternoon.
So the PPI was an indicator released after the Korean market had closed that day.
US PPI rose 1.3% from the previous month on a seasonally adjusted basis, and the year-over-year rate was 6.3%. The Bureau of Labor Statistics explained that the monthly increase was the biggest since March 2022.
This data matters for interpreting later US rates and global market moves, but given the order of events, it can't be used as advance information that caused the October 15 plunge in the Korean market.
You need to read the CPI that Korean investors could check before the open and the PPI that came out after the close separately.
What to Watch in the Market Next
It's too early both to judge that the AI industry is finished based on this drop alone and to assert that a rebound is coming soon because prices fell so much.
Stock prices reflect not only corporate earnings but also rate expectations, investors' appetite for risk, large-cap supply and demand, and even worries about expanding investment through borrowing.
In the coming trading days, four things are worth watching. First, whether the KOSPI and KOSDAQ set new lows or narrow their losses.
Second, whether the moves of big chip stocks like Samsung Electronics and SK hynix continue in the same direction as the overall market. Third, whether foreign selling eases. Fourth, whether the US price and rate outlook worsens further.
Well... you all know this already, right? But the Korean market can't help it. It's affected by external variables far too much.
If prices rise again, rate cuts can be delayed, and AI investment may fail to generate cash flow as quickly as expected. It seems we'll need to keep a close eye on the earnings of AI-related companies, the rate outlook, and money flows from here.
This isn't investment advice. You know that, right? Let's each protect our own hard-earned money.
That's all for this post. See you next time.
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🌐 Korean version: 코스피 7.57% 급락, 10월 15일 한국 증시에서 무슨 일이 있었나
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