South Wales Fire and Rescue Service, police and ambulance crews were called to Cowbridge Road East in the Canton area of the city at 10.17am
Three people were rescued from the flat, the fire service said.
Two fire crews from Cardiff Central, appliances from 1 and Roath fire stations and an aerial ladder platform were called to the scene.
A specialist hazardous area response team, a rapid response vehicle and two road ambulances also attended, with doctors from the emergency medical retrieval and transfer service.
