The scorching hot conditions are set to continue into Monday.
The daily high is forecast to range between 30 and 35 degrees Celsius in most parts of the country and reach 37 degrees in Seoul.
Showers are expected in the provinces of Gyeonggi-do and Gangwon-do during the afternoon.
And Typhoon Francisco is on its way to the Korean Peninsula.
Forecasters say the eighth typhoon of the season is currently moving northwest from Japan, and is likely to make landfall on Korea's southern coast on Tuesday evening.
Francisco is classfied as a relatively small typhoon, and is expected to weaken after passing through the waters between Korea and Japan.