The reading, logged at 1:26 p.m., pushed the country past 42 degrees for the first time in 122 years and marked the third straight day that a national record had fallen. Yangsan has now topped 40 degrees for five consecutive days.
Other cities in the southeast baked alongside it.
Gimhae and the North district of Busan both reached 40.6 degrees, while Gwangyang in South Jeolla province hit 40.3 degrees and Gyeongju climbed to 40.2 degrees.
The Korea Meteorological Administration attributed the searing heat to twin high-pressure systems, the North Pacific and Tibetan highs, blanketing the peninsula at once, combined with intense sunshine, a foehn wind effect and Yangsan's basin terrain.
As of Sunday, cumulative heat-related illnesses since mid-May reached about 1,781 cases nationwide, including 13 deaths, according to the Korea Disease Control and Prevention Agency.
Crowds fled to the coast and to water parks, packing beaches such as Busan's Haeundae, where wait times at some attractions stretched to more than three hours.
Farmers, meanwhile, hauled water to parched fields and set out ice for livestock, as tens of thousands of poultry and farm animals perished across the south.
In the capital, the weather agency upgraded the last remaining district to a heat wave warning at 11 a.m., placing all of Seoul under the alert, while a tropical night advisory entered its ninth consecutive day. The city expanded its response task force to eight teams and checked in on hundreds of vulnerable elderly residents.
"With the heat wave dragging on, the risk of harm to citizens from illnesses such as heat stroke is growing," said Choi Jin-seok, head of Seoul's disaster and safety office. "We will operate our response system so that heat measures work without a gap in the field."
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