The two-alarm blaze was reported around 7:50 a.m. in an apartment complex at 1355 Sir Francis Drake Blvd.
Authorities managed to assist one resident who left the second-story unit where the fire started, but heavy smoke and flames prevented officers from entering the unit, police said. That resident was taken to Marin General Hospital to be treated for injuries that were not considered life threatening.
Another adult resident was later found dead inside the unit, according to police. The victim's name is being withheld pending notification of family, said police Sgt. Robert Schneider.
The blaze also spread to a third-story unit, but residents of that apartment were not home at the time of the fire, according to Schneider.
Ross Valley firefighters were assisted by personnel from neighboring fire departments and managed to bring the fire under control by 8:25 a.m. Sir Francis Drake Boulevard was closed in the area for about 45 minutes as crews worked to combat the flames.
The fire caused an estimated $200,000 of damage.
The cause of the fire is under investigation by the San Anselmo Police Department, Ross Valley Fire Department and Marin County coroner's office.