The fire began at 4 p.m. Saturday and spread to residential areas in Pohang, Yonhap News Agency reported.
A 79-year-old man died while trying to get out of a burning house, and 14 other victims suffered from smoke inhalation, Yonhap said.
About 53 houses were destroyed in the fire. More than 100 people were evacuated.
Police said they were questioning a 12-year-old boy who may have started the fire. He allegedly set fire to a pile of dry leaves at the foot of the residential hill and told authorities after the fire began to spread, police said.
Authorities said Sunday morning the fire was almost put out through the efforts of about 2,000 firefighters and others who worked the blaze.
Meanwhile, another fire in the farming village of Ulsan that had destroyed about two dozen buildings and injured three people was being fought Sunday by about 4,400 firefighters, soldiers and government workers, Yonhap said.