The firemen, five Chilean nationals and one Portuguese, were attempting to extinguish a blaze in Famalicao when the wind apparently changed direction, fanning the flames.
"The firefighters became surrounded by the flames and were not able to flee," said Antonio Gualdino, duty officer for Portugal's National Firefighter and Civil Protection Service.
They had travelled from the Portuguese capital, Lisbon, to Famalicao to investigate the incident, Gualdino said.
More than 150 firefighters, backed by 28 vehicles and three water-dropping aircraft, were at the scene of the blaze, which broke out near the town of Famalicao early on Sunday afternoon amidst scorching temperatures.
The five Chilean firefighters belonged to Afocelpa, a privately run firm which offers firefighting and forest fire prevention services to Portugal's three largest pulp and paper firms.
Police have also detained a man whom they suspect of arson, local television reported.
The forest fire was one of six burning out of control across Portugal on a day when temperatures hit 40 degrees Celsius (104 degrees Fahrenheit) in some regions.
Portugal had suffered severe forest fires last summer, when flames killed 16 people and destroyed 325,226 hectares (803,308 acres) of forest and agricultural land.