The blaze erupted mid-morning on Thursday, sending thick clouds of smoke into the sky above the city.
It is unclear what caused the fire, although initial reports say sparks from welding work may have started the blaze.
Officials at the Capital Security Department said an investigation was under way and they would not release any details until that was complete.
Civil defence units evacuated construction staff from the site, and emergency officials say no injuries have been reported.
Police blocked roads around the site, causing traffic gridlock in much of the capital's downtown area.
At one point blazing materials could be seen falling into the street below and it took two hours to bring the fire under control and put it out.
The building is believed to be part of the al-Nasr Tower complex close to the seafront Corniche area of the city.
Doha is in the midst of a frenzied construction boom on the back of surging economic growth and preparations for the city to play host to the Asian Games in 2006.
Most of the manual construction work is carried out by migrant labourers brought in from South Asia.