In the regular Korean market session on August 18, 2026, the KOSPI fell 1.55% to 6,869.83 points, while the KOSDAQ declined 3.52% to 834.20 points. Both indexes closed below their intraday highs, and SK Hynix was the only tracked stock to rise, showing a closing pattern that differed from the indexes.
A Day When the Closing Decline Outweighed the Intraday Rebound
| Index | August 18, 2026 Regular-Session Close | August 18, 2026 Change |
|---|---|---|
| KOSPI | 6,869.83 points | -108.11 points (-1.55%) |
| KOSDAQ | 834.20 points | -30.45 points (-3.52%) |
The KOSPI began the August 18, 2026 regular session at 7,127.77 points and rose as high as 7,216.62 points intraday, but fell to as low as 6,788.78 points before closing at 6,869.83 points. The close was 257.94 points below the open and 346.79 points below the intraday high. The range between the intraday high and low was 427.84 points. KOSPI closing data
The KOSDAQ opened at 866.59 points on the same trading day, reached a high of 872.59 points, then fell to 827.97 points and ended the regular session at 834.20 points. The close was 32.39 points below the open and 38.39 points below the high, while the intraday range was 44.62 points. The KOSDAQ’s decline was 1.97 percentage points greater than that of the KOSPI. KOSDAQ closing data
The editorial interpretation based on closing data is that weakness was more pronounced in the KOSDAQ that day. However, the simultaneous declines in the two indexes and the difference in their decline rates alone do not establish that a specific sector or investor group directly caused them.
Only SK Hynix Rose; Samsung Electronics Fell More Than 2%
SK Hynix closed at 1,662,000 won on August 18, 2026, up 17,000 won, or 1.03%, from the previous trading day. Compared with the KOSPI’s 1.55% decline on the same day, the relative performance gap was 2.58 percentage points. The open was 1,713,000 won, the high was 1,792,000 won, the low was 1,640,000 won, and regular-session volume was 8,220,532 shares. The close was 51,000 won below the open and 130,000 won below the high. SK Hynix closing data Previous analysis is available at the SK Hynix stock hub.
Samsung Electronics ended the regular session at 268,500 won on August 18, 2026, down 6,000 won, or 2.19%, from the previous trading day. It traded between an open of 284,500 won, a high of 288,000 won, and a low of 265,000 won, with volume of 41,403,610 shares. The close was 16,000 won below the open and 3,500 won above the low. Samsung Electronics closing data Past analysis is available at the Samsung Electronics stock hub.
Collected premarket articles identified whether strength in the U.S. memory sector would translate into buying demand for major domestic semiconductor stocks, including Samsung Electronics and SK Hynix, as a key variable to watch. This was an observation and interpretation presented in the articles, and the August 18, 2026 regular-session closing data alone cannot establish the cause of the two stocks’ divergent movements. Original article
Hyundai Motor Fell 3.97%, While SK Square Was Also Weak
Hyundai Motor closed at 435,000 won on August 18, 2026, down 18,000 won, or 3.97%, from the previous trading day. This was a decline 2.42 percentage points greater than the KOSPI’s -1.55% decline on the same day. The open was 459,500 won, the high was 468,500 won, the low was 427,000 won, and regular-session volume was 1,676,362 shares. The close was 24,500 won below the open and 33,500 won below the intraday high. Hyundai Motor closing data Related records are available at the Hyundai Motor stock hub.
SK Square ended the regular session at 1,135,000 won on August 18, 2026, down 19,000 won, or 1.65%, from the previous trading day. It opened at 1,207,000 won and traded between a high of 1,265,000 won and a low of 1,117,000 won, with regular-session volume of 1,094,972 shares. The close was 72,000 won below the open, 130,000 won below the high, and 18,000 won above the low. SK Square closing data Previous analysis is also available at the SK Square stock hub.
The editorial interpretation is that Hyundai Motor’s -3.97% was the largest decline among the tracked stocks, while SK Hynix’s +1.03% was clearly different from the other three stocks. Relative performance alone does not establish sector-level trading flows or company-specific factors as the cause.
Institutional Net Selling and News Interpretations Should Be Considered Separately
In investor-flow data for August 18, 2026, individuals were net buyers of 7,415억원 and foreigners were net buyers of 914억원, while institutions were net sellers of 7,951억원. Institutional net selling and declines in the KOSPI and KOSDAQ occurred on the same trading day, but this coincidence alone does not establish institutional flows as the direct cause of the index declines. Investor-flow data for August 18, 2026
A NewsPim report cited institutional selling as the backdrop to the KOSPI’s move from the 7,200 level into the 6,900s that day. This was the causal explanation presented in the report; the closing prices and investor-flow figures in this article should be viewed separately as facts confirmed by regular-session prices and investor-flow data, respectively. Original NewsPim article
Three Items to Check in the Next Regular Session
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At the next regular-session close, it is necessary to check how the 81.05-point gap between the KOSPI’s August 18, 2026 close of 6,869.83 points and that day’s low of 6,788.78 points changes, as well as the 6.23-point gap between the KOSDAQ’s close of 834.20 points and low of 827.97 points.
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It is necessary to watch whether SK Hynix’s August 18, 2026 gain of +1.03%, relative to Samsung Electronics’ -2.19%, Hyundai Motor’s -3.97%, and SK Square’s -1.65%, continues at the next regular-session close. This is a comparison item, not a basis for determining a future direction.
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It is necessary to check how the direction of investor flows—individual net buying of 7,415억원, foreign net buying of 914억원, and institutional net selling of 7,951억원 on August 18, 2026—changes in the next regular-session tally. Changes in investor flows and price movements should be observed together, but a causal relationship cannot be established from data for a single trading day alone.