A karate training group in Nichinan City, Miyazaki Prefecture, holds the event every New Year's Day. About 30 boys and girls took part.
The children started by performing karate early in the morning at a shrine, praying to improve their skills.
They moved to the nearby coast of the Pacific Ocean, where they practised karate in water up to their knees.
The overnight low in the city was 6.2 degrees Celsius.
After the practice, the children sipped hot sweet red bean soup around a bonfire to warm themselves.
A boy said he felt the seawater was slightly warm, and he enjoyed practising punches.