Flames could be seen rising from the roof of the building in the centre of the city early on Sunday, while a dark plume of smoke was visible from miles away.
It was not immediately clear if anyone was in the building.
Initial reports indicate the blaze started in the early hours of Sunday in the third-floor offices before spreading towards a gym, according to Jean-Pierre Smith, a Cape Town mayoral committee member responsible for safety and security.
The roof area also caught alight, as did the National Assembly building, he added.
TV footage showed emergency services at the scene.
Mr Smith said that included six firefighting appliances and about 36 firefighters.
City of Cape Town Fire and Rescue Service spokesman Jermaine Carelse told local media the fire started in the early hours of Sunday in the third-floor offices and spread to the National Assembly chamber.
He said no people had been injured in the fire, which was reported by security guards.
Several hours after the blaze started, thick smoke could still be seen pouring from one of the several buildings that make up the parliament complex as emergency services sprayed water inside.
Patricia De Lille, public works and infrastructure minister, told reporters the blaze was under control.
She said it was currently burning in the National Assembly chamber but had been contained in other parts.
The cause of the blaze is not yet known.
Additional reporting by agencies