The figure surpassed the previous early-August record of $15.6 billion set in 2022.
Both periods contained seven working days, with average daily exports also rising 45.3 percent to $3.04 billion.
Semiconductor exports soared 155.4 percent to $9.95 billion, marking a record for the early-August period.
Chips accounted for 46.8 percent of total exports, up 20.1 percentage points from a year earlier, according to the Korea Customs Service.
Exports excluding semiconductors rose about 5.4 percent to $11.33 billion, according to an AJP calculation based on preliminary customs data.
The latest figures extend a broader export rally fueled by global investment in artificial-intelligence infrastructure, which has lifted demand for memory chips and data-center equipment.
Petroleum-product exports rose 65.3 percent to $2.00 billion, while computer peripherals surged 139.5 percent to $587 million.
The gains were partly offset by an 80.9 percent plunge in passenger-car exports and declines of 58.2 percent in ships and 36.3 percent in automobile parts.
Shipments to China more than doubled to $6.75 billion, rising 134.8 percent from a year earlier.
Exports to Vietnam increased 45.4 percent to $2.39 billion, while those to the European Union climbed 57.2 percent to $1.50 billion.
Exports to the United States edged down 0.2 percent to $2.05 billion, contrasting with the sharp gains recorded in most other major markets.
China, Vietnam and the United States together accounted for 52.5 percent of South Korea's outbound shipments.
Imports rose 23.1 percent to $19.49 billion.
Semiconductor imports climbed 90.8 percent and chipmaking-equipment imports gained 77.3 percent, while lower crude-oil and gas purchases pulled total energy imports down 13.8 percent.
South Korea posted a trade surplus of $1.80 billion, reversing a $1.18 billion deficit in the same period last year.
The customs agency cautioned that the preliminary figures cover a short period and may be affected by shipment schedules.
AJP Takeaways
South Korea's exports jumped 45.3 percent to a record $21.29 billion in the first 10 days of August.
Semiconductor exports surged 155.4 percent to $9.95 billion and accounted for 46.8 percent of total exports.
Exports excluding semiconductors rose by about 5.4 percent to $11.33 billion.
Exports to China rose 134.8 percent to $6.75 billion, while shipments to the United States slipped 0.2 percent to $2.05 billion.
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