Crews from Elm Lane and Rivelin fire stations were sent to the blaze, at Willington Road, Firth Park, around 11.30pm last night.
Officers believe the ground floor flat had been ablaze for some time when they arrived to find the living room in flames.
It was unoccupied and no one was injured .
Firefighters using hosereels were on the scene for around 90 minutes. The cause is unknown.