The benchmark KOSPI traded at 6,808.63 points at around 9 a.m., down 0.64 percent from the previous session. The junior KOSDAQ also fell 3.53 percent to 811.22, with losses considerably steeper among smaller growth stocks.
The decline came after the KOSPI surged 5.89 percent the previous day, powered by a rebound in semiconductor heavyweights including SK hynix, after the chipmaker announced a record 40 trillion won (US$28 billion) share buyback plan.
But major chipmakers bucked the broader decline in early trading. Samsung Electronics rose 0.18 percent to 271,500 won, while SK hynix gained 1.42 percent to 1,715,000 won. Samsung Electronics preferred shares climbed 3.61 percent to 198,100 won as of 9:35 a.m.
Elsewhere among major stocks, losses were widespread. Samsung Electro-Mechanics dropped 4.94 percent to 1,327,000 won, LG Energy Solution fell 2.37 percent to 349,500 won and Hyundai Motor lost 1.38 percent to 411,750 won. Hanwha Aerospace slid 6.26 percent to 1,094,000 won, while HD Hyundai Heavy Industries declined 3.26 percent to 460,000 won.
Financial stocks were mixed, with KB Financial rising 1.06 percent to 161,700 won and Shinhan Financial Group gaining 1.48 percent to 102,600 won. Samsung Life Insurance fell 1.01 percent to 294,000 won.
The selloff was sharper on the KOSDAQ, where all 10 of the largest stocks shown in early trading were lower. Biotech platform developer Alteogen fell 5.74 percent to 320,000 won, while battery materials group EcoPro dropped 4.43 percent to 84,200 won and cathode-material maker EcoPro BM lost 5.26 percent to 108,100 won. Robot maker Rainbow Robotics slid 5.51 percent to 445,500 won.
Semiconductor-related equipment and component makers also retreated. Jusung Engineering, which makes semiconductor manufacturing equipment, fell 1.35 percent to 174,900 won, while semiconductor and display equipment maker Wonik IPS dropped 3.25 percent to 110,200 won.
Leeno Industrial, a maker of semiconductor testing components, declined 4.03 percent to 66,700 won, and laser-based semiconductor equipment maker EO Technics lost 2.53 percent to 404,500 won. Biotech companies were also weak, with HLB down 4.26 percent at 38,200 won and bispecific-antibody developer ABL Bio tumbling 6.67 percent to 77,000 won.
Early trading showed investors turning cautious again. Retail investors bought a net 70.8 billion won ($50.7 million) of KOSPI shares, while foreign and institutional investors sold a net 20.6 billion won and 109.1 billion won, respectively. Program trading recorded net selling of about 183.9 billion won.
The cautious start followed a broad retreat on Wall Street overnight, where higher Treasury yields and signs of weaker U.S. consumer spending weighed on stocks.
The Dow Jones Industrial Average fell 1.32 percent, to 52,759.21 on Thursday. The S&P 500 dropped 0.87 percent to 7,641.16, while the Nasdaq Composite lost 1.00 percent to 26,067.17.
Concerns about consumer spending deepened after Walmart, the largest U.S. retailer, reported that sales at its established U.S. stores rose just 2.6 percent in the second quarter, the slowest pace in six years. Its shares plunged 9.15 percent as high fuel prices added to worries that consumers could cut back on spending.
Bond yields added to the cautious mood. The benchmark 10-year U.S. Treasury yield climbed 5 basis points to around 4.70 percent, while the 30-year yield rose about 4 basis points to 5.24 percent.
The moves came just a day after yields briefly eased when the U.S. Treasury announced plans to expand its buyback of long-term government bonds. The program allows the Treasury to purchase bonds already trading in the market, making them easier to buy and sell.
The measure helped calm global bond markets and supported Thursday's rebound in Korean equities, but investors remain concerned that heavy government borrowing and more debt issuance by big tech companies to fund artificial intelligence (AI)-related investment could keep yields elevated.
Higher bond yields can weigh particularly heavily on technology and other growth stocks by increasing borrowing costs and reducing the value investors place on future earnings.
Oil prices posed another risk, climbing more than 2 percent amid heightened tensions surrounding Iran. October Brent crude futures settled at $93.78 a barrel, while September West Texas Intermediate crude ended at $87.83.
Friday's early decline marked a renewed test for the South Korean market after the previous day's powerful rebound, with investors watching whether semiconductor momentum can withstand another bout of pressure from higher global yields and oil prices.
Meanwhile, the won strengthened, trading at 1,385.10 against the U.S. dollar as of 9 a.m., compared with 1,392.60 the previous day.
AJP Takeaways
• South Korea's KOSPI fell 0.64 percent to 6,808.63 as of 9:16 a.m. on Aug. 21, 2026, giving back part of its 5.89 percent surge on Aug. 20 as higher U.S. Treasury yields, rising oil prices and Wall Street losses weighed on sentiment.
• Samsung Electronics rose 0.18 percent to 271,500 won and SK hynix gained 1.42 percent to 1,715,000 won, bucking the broader market decline after SK hynix's 40 trillion won ($28 billion) share buyback and cancellation plan helped fuel the previous session's rebound.
• U.S. Treasury yields climbed on Aug. 20, with the 10-year yield reaching about 4.70 percent and the 30-year yield 5.24 percent, despite the U.S. Treasury's move to expand buybacks of long-term government bonds.
• Walmart shares plunged 9.15 percent on Aug. 20 after the largest U.S. retailer reported its slowest second-quarter U.S. comparable-sales growth in six years, adding to concerns that high fuel prices could curb consumer spending.
• The South Korean won strengthened to 1,385.10 per U.S. dollar as of 9 a.m. on Aug. 21, 2026, from 1,392.60 the previous day.
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