The index rose 3.4 percent 7,031.09, with foreigners the lone net buyers as individuals and institutions sold into the advance.
Foreigners bought a net 680.5 billion won of shares in early trade, against individual selling of 640 billion won as retailers took the foreign return as a cue to exit after volatile bouts.
SK hynix led the large caps, climbing 5.6 percent to 1,933,000 won ($1,291). Samsung Electronics rose 4 percent to 271,000 won ($183), extending gains as brokerages lifted their price targets on the stock.
Samsung Biologics advanced 2.33 percent to 1,404,000 won after reporting a 30-percent gain in second-quarter revenue and 23 percent in operating profit.
Samsung Electro-Mechanics jumped 7.4 percent to 1,444,000 won ($977), the steepest move among the index heavyweights.
Advancers outnumbered decliners 678 to 163 on the main board.
The tech-heavy KOSDAQ added 1.9 percent to 765.3, running its own advance alongside the main index in Seoul.
Traders read the surge as a bet on a strong earnings season, with brokerages flagging the prospect of another quarterly beat at Samsung.
In Tokyo, the Nikkei 225 rose 0.9 percent. The won traded at 1,477.5 per dollar, marginally stronger than the previous session.
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