Military helicopters have been mobilised to try and douse the flames raging in forested areas of Ofunato in the northern region of Iwate.
Aerial footage from public broadcaster NHK showed the bright orange blaze reducing several houses to charred frames.
"One burned body was discovered and transferred to a nearby police station," a local police official told AFP.
Nearly 600 nearby residents have been evacuated, according to the Ofunato municipality.
As of Thursday morning, city officials estimated that at least 84 buildings had been damaged in the fire.
Ofunato's mayor Kiyoshi Fuchigami described the fire as "large-scale" on Wednesday evening, saying around 600 hectares (1,500 acres) of land -- three times the size of Monaco -- had been burned.
The cause of the blaze remained unknown, he added.
There were about 1,300 wildfires across Japan in 2023, concentrated in the February to April period when the air dries out and winds pick up.
Ofunato has seen only 2.5 millimetres (0.1 inches) of rainfall this month -- on course to fall far below the previous record low for Feburary of 4.4 millimetres in 1967.
And last year was Japan's hottest since records began, mirroring other nations as ever-rising greenhouse gas emissions fuel climate change.
In the case of the latest Iwate fire, "strong wind" from the west helped spread a series of smaller wildfires in the same area, Fuchigami said.
One evacuee, a 45-year-old woman, told NHK that the fire had been approaching her house when she returned from work.
"I was relieved that my kids were safe," she said.
Another, a 32-year-old man, told the broadcaster it was "my first time to see a series of fires happen in such rapid succession".
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