Sea temperature in South Korea

The sea in South Korea peaks in July, when the water averages around 24°C.

The main swimming season runs from June – September, when surface temperatures stay comfortable for extended sessions.

On average, 4 months a year sit above 20°C — the threshold most swimmers describe as pleasant without a wetsuit.

Shoulder months like May and October are noticeably cooler but often quieter, with fewer crowds and steadier weather.

Outside peak season the water drops toward 14°C, so a 3/2mm shorty or full wetsuit makes long swims much more comfortable.

The coldest month is typically February at around 12°C, when cold-water swimmers dominate the coast.

Long-term climatology for South Korea shows an annual mean sea temperature of 18°C, ranging from 12°C in winter to 24°C at the summer peak.

The seasonal swing is roughly 12°C between the coldest and warmest month — a useful yardstick when comparing years or checking whether the current sea is running above or below average.

Recent decades have trended slightly warmer at South Korea, in line with regional sea-surface temperature records for the wider basin.

Monthly averages below are drawn from multi-decadal climatology and are updated as new observations arrive.