The Korea Meteorological Administration (KMA) issued the advisory as the mercury soared to 34.4 degrees Celsius in the afternoon in Seoul. It also issued heat advisories for many other parts of the country.
The advisory is issued when the highest "feels-like" temperature is expected to climb above 33 degrees Celsius for more than two consecutive days.
The KMA advised people to "refrain from outdoor activities as much as possible during daytime hours."
Sweltering heat is expected to persist for the time being, with daytime highs hovering around 35 degrees in most part of the country.
Meanwhile, Jeju Island is expected to come under the influence of a monsoon front. The KMA forecast rain to begin late Wednesday night or early Thursday morning with expected precipitation of about 100 millimeters.
But the KMA has not yet said when the rainy season arrives in Seoul. Korea typically sees an average of 17.7 rainy days during the monsoon season.
