At about 12.30am, emergency services were called to Military Road in Semaphore South.
Patrols quickly arrived and saw flames coming from the unit.
An elderly occupant was at the front door but she was unable to get out due to mobility issues, police said.
An officer entered the burning home and carried the 90-year-old woman out of the inferno and down the street, away from the fire.
Occupants of nearby units were also evacuated, but the fire was contained to just the one residence.
The 90-year-old woman suffered non-life threatening smoke inhalation and she was taken to hospital for observation.
The police officer was assessed at the scene for minor smoke inhalation.
The fire is non-suspicious, however, the cause is undetermined at this time.