The combined area affected amounts to the third largest recorded in Japan, where forest fires have intensified in recent years.
The first fire broke out on Wednesday in a mountainous area and then a second nearby threatened residential districts in the town of Otsuchi in Iwate Prefecture.
Hilly terrain, dry weather and winds are hampering containment efforts, a fire department official told reporters.
By Saturday, the fires had scorched about 730 hectares, prompting evacuation orders covering 1541 households and 3233 people - about a third of Otsuchi's population.
The town is scarred by the memory of one of Japan's worst disasters, the March 2011 earthquake and tsunami when it lost nearly a tenth of its population.
"Even during the 2011 disaster, this area didn't burn. There was a tsunami but we had no fire here," said Taeko Kajiki, 76, a former nurse, who was among those who have been evacuated since Friday.
She said she had stayed up all night watching the red glow of the flames and had packed her bankbook and medical cards as well as the turtle she has kept as a pet since 2010.
The firefighters on the ground were supported by helicopters from several prefectures and Japan's Self Defense Forces, which carried out aerial water drops to try to halt the advance of the fires.
"With the land so dry, fires keep igniting. We put one out, then race to extinguish another, over and over again," said Masashi Kikuchi, a 37-year-old volunteer firefighter, who moved to a house on higher ground after losing his home to the 2011 tsunami.
Eight buildings, including one residential home, have been damaged or destroyed so far, though no injuries or fatalities have been reported, authorities said.
"I can't let people lose their homes again after losing them once to the tsunami," Otsuchi Mayor Kozo Hirano told reporters. He said the town would seek help from other authorities and provide services, such as hot baths, to help ease stress among residents.
While Japan has experienced relatively few wildfires compared with other parts of the globe, climate change has increased their frequency, especially as the early spring months before the humid rainy season have been hot, dry and with winds that can whip up flames.
The Japan Meteorological Agency said no rain was forecast for the region over the coming week.
According to official figures, the amount of land on fire around Otsuchi is second only to the major forest fire in Ofunato in 2025, which consumed about 3370 hectares, and the Kushiro fire in 1992, which burned 1030 hectares.
