Sudden rainfall of 50-150 milliliters poured down in Seoul from 8:30 a.m., catching commuters off guard.
The local weather agency cited the influence of Typhoon Hato and cold air descending from northern regions as causes of the heavy rain in the nation.
The rain is expected to continue through Thursday, with some areas expected to see as much as 200 milliliters of torrential rain.
In the southern regions of Jeolla Province and Jeju Island, the hot weather is expected to linger, accompanied by occasional squalls in the afternoon.
Wednesday marked Cheoseo, one of the 24 seasonal divisions that signals the start of fall.