The KOSPI on May 18 triggered a sell sidecar for a second straight session after plunging more than 5 percent early in the session, but later recouped most of its losses to regain the 7,500 level.

The KOSPI closed down 22.86 points, or 0.31 percent, at 7,516.04. It opened down 49.89 points, or 0.67 percent, at 7,443.29 and at one point slid to 7,142.71. The index later rebounded into the 7,600s before ending slightly lower.

The Korea Exchange triggered a sell sidecar at 9:19:22 a.m. after KOSPI 200 futures fell 5.13 percent from the previous session to 1,112.46, temporarily halting program sell orders.

Retail investors were net buyers of 2.2091 trillion won and institutions bought a net 1.3912 trillion won. Foreigners were net sellers of 3.6496 trillion won.

Foreigners have extended net selling for 8 straight sessions since May 7, the day after the KOSPI broke above 7,000, adding to supply pressure even as the index rebounds.

Top market-cap stocks were mixed. Samsung Electronics ended up 3.88 percent at 281,000 won and SK Hynix rose 1.15 percent to 1.84 million won. Samsung Electro-Mechanics gained 2.08 percent, Doosan Enerbility added 1.17 percent and Samsung C&T rose 0.76 percent.

SK Square fell 0.46 percent, Hyundai Motor dropped 5.29 percent, LG Energy Solution slid 2.16 percent and HD Hyundai Heavy Industries declined 3.91 percent.

The Kosdaq closed down 18.73 points, or 1.66 percent, at 1,111.09.

In Seoul's foreign exchange market, the won ended at 1,499.70 per dollar, down 1.70 won from the previous session.