The blaze in the 1400 block of Marshall Road in Alpine erupted for unknown reasons shortly before 2 p.m., according to sheriff's officials.
Emergency personnel arrived to find the home engulfed in flames, Lt. Robert Smith said. It took engine crews about 10 minutes to get the fire under control.
The victim, whose name was not immediately available, was pronounced dead at the scene.
Sheriff's arson detectives were called in to investigate, which is routine in fires that involve fatalities, the lieutenant said.