As of Monday morning, heatwave advisories were issued in Seoul, Busan and most of Gyeonggi-do and Gangwon-do Provinces.
Heatwave warnings are issued when the daily high is expected to stay above 35 degrees Celsius for two or more days in a row; advisories go out when it's staying above 33.
The advisories were also issued in most parts of the central Chungcheong-do provinces, with the southern city of Gwangju put under a heatwave warning.
South Korea's weather agency warns that too much exposure to heat can cause several health problems such as heat exhaustion and heat stroke.
Heat exhaustion happens when the body overheats. It causes symptoms such as muscle cramps, excessive sweating and cool skin. Staying hydrated is the key to preventing it. If heat exhaustion goes unaddressed, it can lead to heat stroke, which is a much more serious condition and requires emergency treatment.
Heat stroke causes loss of consciousness, organ failure and can even be deadly. The most important thing to do is to lower the body's core temperature.
The agency says the country will likely see heatwaves continue in the coming weeks, so people should stay indoors and drink plenty of water.
Choi Si-young, Arirang News.