The fire, which started yesterday, is still burning out of control and is already one of the deadliest in Portugal in decades.
It hit the mountainous area of Pedrogao Grande, 200km south-east of Lisbon, amid an intense heat wave fanned by strong winds.
Several firefighters are among the 59 people injured.
It is not yet known what caused the fire, which has also burnt down houses in the area.
"Unfortunately this seems to be the greatest tragedy we have seen in recent years in terms of forest fires," said Prime Minister Antonio Costa.
The death toll could rise further, he said.
Secretary of State for the Interior Jorge Gomes earlier said that three people died from smoke inhalation and 16 died in their cars on the road linking Figueiró dos Vinhos to Castanheira de Pera.
About 60 forest fires broke out across the country overnight, with 1700 firefighters working to put them out.
The flames spread "with great violence" on four fronts, Mr Gomes said.
Spain has sent two water-bombing planes to help tackle the fires.
Portugal has been experiencing a heatwave, with temperatures over 40C in some areas.
Photo: AFP Firefighters rest during a wildfire at Penela, Coimbra, central Portugal.