Hongcheon in Gangwon Province saw the mercury reach 40.3 Celsius (105 degrees Fahrenheit) at 2.20 p.m. Wednesday, according to the Korea Meteorological Administration, while most local media outlets claimed the temperature rose as high as 41C -- around 10 degrees above normal for this time of year.
The country's previous high had been 40C in Daegu on Aug. 1, 1942.
South Korea has been collecting weather records since the early 1900s, and this summer has also been remarkable because the 16-day rainy season was only half as long as the average duration.
Seoul's official temperature hit 39.6C (103F) Wednesday, a new high for the capital, replacing 38.4C measured in 1994.
The city's record morning temperature of 30.3C at 6.30 a.m. Thursday fueled speculation that history might be rewritten again later in the day.
Local authorities have been issuing regular public health warnings as thousands of people have suffered heatstroke under a relentless blanket of humidity.
According to the Korea Centers for Disease Control and Prevention, 28 people have died due to the heat this summer as of July 30.
More about: South-Korea