Samsung Electronics and SK hynix led the broad rebound as foreign investors returned after uncertainties over rate decisions by the U.S. and Japanese central banks were removed.
The benchmark KOSPI closed at 6,894.23, up 2.66 percent from the previous session. The index opened 2.54 percent higher at around 6,885 and climbed as high as 6,914.08 during the session.
Institutions bought a net 1.51 trillion won ($1.09 billion) of KOSPI shares, while foreign investors bought 419.1 billion won ($303 million). Retail investors sold a net 3.59 trillion won ($2.59 billion).
The rebound came as investors absorbed the Fed's first rate increase in three years. The U.S. central bank raised its benchmark rate by 0.25 percentage point to a range of 3.75 percent to 4.00 percent Wednesday and signaled that further tightening remains possible this year.
The Bank of Japan (BOJ) on Friday raised its benchmark interest rate by 25 basis points to 1.25 percent in a 7-2 vote at the end of its two-day monetary policy meeting, resuming tightening after a three-month pause. The increase took the policy rate to its highest level since 1995.
Samsung Electronics and SK hynix led the rally. In after-market trading as of 4:20 p.m., Samsung Electronics was up 3.37 percent at 261,000 won. SK hynix was up 6.13 percent at 1,852,000 won. Samsung Electronics' preferred shares gained 1.91 percent to 197,000 won.
The gains followed a strong session for U.S. chip stocks. The Philadelphia Semiconductor Index rose 3.14 percent overnight as Intel, Advanced Micro Devices and other semiconductor shares rallied on continued optimism over artificial intelligence demand.
Other technology shares also advanced. SK Square climbed 6.55 percent to 1,074,000 won, while Samsung Electro-Mechanics rose 4.66 percent to 1,392,000 won.
Among other large-cap stocks, Hyundai Motor gained 0.83 percent to 366,000 won and Samsung Biologics edged up 0.14 percent to 1,398,000 won.
Battery maker LG Energy Solution slipped 0.14 percent to 364,500 won. KB Financial fell 2.34 percent to 175,000 won, while Samsung C&T lost 0.70 percent to 353,000 won.
Lower oil prices and U.S. Treasury yields eased some of the pressure that had weighed on equities earlier in the week.
West Texas Intermediate crude fell about 2.3 percent overnight, and Brent crude dropped about 3 percent. Expectations of additional Saudi supply and easing tensions involving Yemen's Houthi movement reduced concerns over oil disruptions.
The U.S. 10-year Treasury yield also retreated to 4.939 percent as investors reassessed the outlook after the Fed decision.
Still, concerns over prolonged monetary tightening remain. Sixteen of 18 Fed policymakers expect at least one additional rate increase before the end of 2026, according to projections released with the latest decision.
The junior KOSDAQ also closed higher, rising 0.60 percent to 827.12. Institutions bought a net 23.2 billion won of shares and retail investors bought 12.8 billion won. Foreign investors sold a net 46.6 billion won.
Orum Therapeutics plunged 29.96 percent to 37,400 won after Bristol Myers Squibb (BMS) stopped developing ORM-6151, an experimental blood cancer drug it acquired from Orum in 2023.
The decision wiped out up to $80 million in potential future payments tied to the drug's development, weighing on the stock. The company will keep the $100 million upfront payment it already received.
Among other junior stocks, cathode-material maker EcoPro BM was up 1.85 percent at 104,600 won. Semiconductor equipment maker Wonik IPS was up 3.22 percent at 121,800 won, while robotics company Rainbow Robotics was up 1.40 percent at 434,500 won.
Jusung Engineering, which makes semiconductor and display equipment, was unchanged at 204,000 won. Alteogen was also flat at 254,000 won.
Battery-material holding company EcoPro was down 0.97 percent at 81,500 won, while laser equipment maker EO Technics was down 1.62 percent at 455,000 won.
Semiconductor test-component maker Leeno Industrial was up 4.22 percent at 69,200 won. Printed-circuit-board maker Simmtech was down 1.56 percent at 126,200 won, while robotics company Robotis was down 0.97 percent at 307,000 won.
The Korean won traded at around 1,383 per dollar in late trading, compared with 1,382.20 in the previous session.
AJP Takeaways
- South Korea's KOSPI jumped 2.66 percent to 6,894.23 on September 18, 2026, as lower oil prices and U.S. Treasury yields supported a rebound led by semiconductor shares.
- Samsung Electronics was up 3.37 percent at 261,000 won and SK hynix was up 6.13 percent at 1,852,000 won in after-market trading as of 4:20 p.m.
- Orum Therapeutics plunged 29.96 percent to 37,400 won after Bristol Myers Squibb ended development of ORM-6151, removing up to $80 million in potential future payments while allowing Orum to keep its $100 million upfront payment.
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