Firefighters responded to reports of a two-alarm fire at Grove Apartments at 243 Buena Vista Ave. at about 3:30 p.m., Sunnyvale spokesman John Pilger said.
Responding crews upgraded the incident to a three-alarm fire, and about 40 firefighters battled the blaze for about an hour.
"The fire started in the first floor, which was heavily damaged. Then it went up to the second floor and through the roof," Pilger said.
Firefighters used a fire ladder truck to put out small fires that started on neighboring buildings, and the main fire was put out at about 4:30 p.m. The neighboring structures were minimally damaged.
The body of 70-year-old woman, believed to be a resident of Grove Apartments, was found in the burning building, Pilger said.
He said a firefighter also suffered smoke inhalation during the battle and was taken to a local hospital for treatment.
No other injuries were reported.
Investigators are still looking into the cause of the fire, but preliminary results show the blaze was unlikely to be suspicious.
Firefighters from Santa Clara County and Mountain View assisted battling the fire.
The American Red Cross was called to assist any residents displaced by the fire. Pilger was unable to say how many people were displaced.
"There are some relatives that live in other apartments that were not damaged in the complex, and displaced residents might go stay with them," he said.
A damage estimate was not immediately available.