The mercury soared to this year's record of 39.9 degrees Celsius in Gyeongsan, North Gyeongsang Province on Thursday.
Daytime highs also climbed to near 40 degrees in Daegu (38.6), Yeongcheon (38.4) and Gwangju (37.3).
The Korea Meteorological Administration issued a heat warning for Daegu, some parts of North Gyeongsang Province and Jeonju in North Jeolla Province, and a heat advisory for the other areas.
A heat advisory is issued when the daytime high reaches more than 33 degrees for more than two days, and a heat warning when it climbs above 35 degrees.
Sweltering heat will continue for the time being in central parts and inland areas, the weather agency forecast. The daytime high in Seoul was forecast to reach 34 degrees on Friday and 33 degrees on Saturday.
But typhoon Nakri, which is moving northward to the West Sea, is expected to send rain and squalls across the country on Saturday.
Nakri formed in the sea 860 km off Okinawa at around 9 a.m. on Wednesday and developed into a typhoon with a central pressure of 990 hPa and a maximum wind speed of 24 m/s. It is advancing toward the West Sea at a speed of 22 km/h.
The KMA forecast rain for the southern parts including Jeju on Friday, when it comes under the indirect influence of the typhoon. Nakri will make landfall on the resort island on Saturday and indirectly affect the other areas, bringing heavy rains.
The typhoon will likely affect the nation until Tuesday, a weatherman said.