The index had last closed above that threshold on Feb. 27, when it was at 6,244.13, just before the outbreak of the conflict in the Middle East that began with U.S.-led airstrikes on Iran.
The gain was driven by foreign investors, who bought about 550 billion won ($370 million), while the junior KOSDAQ also ended higher, rising 2.72 percent to 1,152.43.
Large-cap stocks mostly closed higher. Among chip-related stocks, Samsung Electronics rose 2.18 percent to close at 211,000 won, and SK hynix gained 2.99 percent to 1,136,000 won. The two accounted for about 40.9 percent of the KOSPI’s total market capitalization, up from 39.9 percent at the previous peak, with their combined market value surging by about 227 trillion won since April 7, just before the U.S. and Iran agreed to a two-week ceasefire.
Their dominance is expected to intensify further, supported by a semiconductor supercycle. Both Samsung Electronics and SK hynix are on track to post record-high operating profits this year, with some forecasts suggesting they could rank among the world’s top three in operating profit next year.
A similar trend is seen globally, with Taiwan’s TSMC accounting for over 40 percent of the country’s stock market.
Other sectors also posted solid gains. In the auto sector, Hyundai Motor climbed 3.36 percent to 508,000 won, while Kia added 1.54 percent to 151,500 won.
Battery and energy shares were largely higher, with LG Energy Solution up 2.00 percent at 408,000 won and SK Inc. rising 3.91 percent to 665,000 won, while Hanwha Aerospace slipped 0.92 percent to 1,509,000 won.
Among biopharmaceutical stocks, Samsung Biologics gained 4.30 percent to 1,602,000 won and Doosan Enerbility rose 4.51 percent to 104,200 won.
Financial sectors also closed higher, with KB Financial Group up 1.48 percent at 158,200 won, Samsung Life Insurance rising 4.88 percent to 258,000 won, and Shinhan Financial Group gaining 1.53 percent to 99,800 won.
Major KOSDAQ stocks also ended broadly higher. Among biopharmaceutical and healthcare shares, Alteogen surged 5.67 percent to close at 373,000 won, and Samchundang Pharm jumped 6.73 percent to 555,000 won. ABL Bio rose 5.07 percent to 163,800 won, HLB climbed 8.77 percent to 68,200 won, and LigaChem Biosciences advanced 6.63 percent to 193,000 won.
In other sectors, Ecopro gained 2.57 percent to 147,900 won, and Ecopro BM added 2.38 percent to 202,500 won. Rainbow Robotics edged up 1.33 percent to 611,000 won, while Koh Young Technology rose 9.74 percent to 104,800 won and Rino Industrial increased 1.08 percent to 112,800 won.
The dollar initially retreated on foreign stock buying, trading at 1,471.80 won in the morning trading, before paring losses to 1,474.20 won in the afternoon, compared with the previous close of 1,481.20.
Elsewhere, Japan's Nikkei 225 rose 0.44 percent to 58,134.24, Hong Kong’s Hang Seng Index advanced 0.37 percent to 25,967.30, China’s Shanghai Composite Index edged up 0.015 percent to 4,027.21.
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