The won strengthened 14.1 won from the previous session to close at 1,397.7 per dollar, compared with Tuesday's close of 1,411.8.
It marked the first daytime close below 1,400 since Sept. 29, 2025, when the currency ended at 1,398.7.
Exporter dollar selling and broader weakness in the U.S. currency supported the won despite heavy foreign selling in Korean equities.
The KOSPI plunged 5.8 percent to 6,471.17 as technology shares came under heavy selling pressure, with the decline triggering a program-trading sidecar during the session.
The South Korean government bonds also strengthened, with the three-year yield falling 4.9 basis points to 3.798 percent and the 10-year yield declining 4.5 basis points to 4.337 percent.
The bond rally began after U.S. Treasury yields eased overnight, while the steep decline in Korean stocks added to demand for government debt.
Gains extended in the afternoon after the Korea Development Institute raised its 2026 growth forecast to 3.2 percent from 2.5 percent, as the revision came in below levels some bond investors had braced for.
Foreign investors also supported the short end of the market, buying more than 7,000 three-year government bond futures by early afternoon even as they remained net sellers of 10-year futures.
Wednesday's decline partly reversed the previous session's bond selloff, when the three-year yield rose 5.1 basis points to 3.847 percent and the 10-year yield climbed 6.9 basis points to 4.382 percent.
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