The index shed 76.68 points to 6,836.27, a fall of 1.1 percent.
The drag was almost entirely one stock. SK hynix rose 1.4 percent, the junior KOSDAQ market gained 1.5 percent, and shipping and non-ferrous metal shares led broad advances across the main board.
Samsung Electronics traded at 265,000 won, down 16,500 won ($192).
The company's board approved a 2026 shareholder return of roughly 90 trillion to 110 trillion won, or 65 billion to 80 billion dollars, after Friday's close. It is about five times the previous domestic record of 20.3 trillion won set in 2020.
Samsung said it would begin with a cash dividend of about 30 trillion won, some 22 billion dollars, in the third quarter, and separately approved 15 trillion won of share purchases for employee compensation.
The size was not the problem.
Everything beyond the third-quarter dividend stays undefined until the board meets again in January, once full-year results are fixed.
Traders read that gap against SK hynix, which has already committed to returning more than half of its free cash flow over the next three years and had announced 40 trillion won of buybacks and cancellations. Samsung set out no comparable forward policy.
SK hynix traded at 1,754,000 won, or about $1,270, up 24,000 won.
The selling ran through the wider Samsung complex, where the payout had been the trade. Samsung Life Insurance, the electronics maker's largest shareholder, dropped 8.2 percent to 301,500 won, or about $218. Samsung C&T fell 7.2 percent to 367,000 won, roughly $266.
Both hold stakes that would have collected a share of the dividend.
Foreign investors sold a net 902.6 billion won of KOSPI shares in the opening stretch, and institutions sold 299.5 billion won. Individuals bought 974.7 billion won.
The KOSDAQ traded at 813.74, up 11.80 points.
In Tokyo, the Nikkei 225 stood at 65,903.48, down 0.2 percent.
The won traded at 1,381.40 per dollar, stronger than Friday.
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