The dead were all adults, Margarida Castro Martins, head of Lisbon's Civil Protection Agency, told reporters on Thursday.
She did not provide their names or nationalities, saying that their families would be informed first.
Another 21 people were injured in Wednesday's crash, including Portuguese people as well as two Germans, two Spaniards and one person each from France, Italy, Switzerland, Canada, Morocco, South Korea and Cape Verde.
The range of nationalities reflected how big a draw the renowned streetcar was for tourists who are packing the Portuguese capital during the summer season.
Portugal observed a national day of mourning on Thursday after the capital's worst disaster in recent history.
Though authorities gave no details about those killed, the transport workers' trade union SITRA said the streetcar's brakeman, Andre Marques, was among the dead.
The 19th-century streetcar is one of Lisbon's big tourist attractions and is usually packed with foreigners at this time of year for its short and picturesque trip up and down one of the city's steep hills.
The streetcar's crumpled wreckage on Thursday was still on the downtown road where it crashed, cordoned off by police.
Detectives photographed the rails and the wreckage on the deserted road.
Officials declined to speculate on whether a faulty brake or a snapped cable may have caused the derailment.
The yellow-and-white streetcar, known as Elevador da Gloria, was lying on its side on the narrow road that it travels on, its sides and top crumpled.
It crashed into a building where the road bends, leaving parts of the mostly metal vehicle crushed.
"It hit the building with brutal force and fell apart like a cardboard box," witness Teresa d'Avo told Portuguese television channel SIC.
She described the streetcar as out of control and seeming to have no brakes, and said she watched passers-by run into the middle of the nearby Avenida da Liberdade, or Freedom Avenue, the city's main thoroughfare.
The crash occurred at the start of the evening rush hour, about 6pm.
Emergency officials said all victims were pulled out of the wreckage in just over two hours.
The streetcar, technically called a funicular, is harnessed by steel cables and can carry more than 40 people, seated and standing.
The service, inaugurated in 1885, goes up and down a few hundred metres of a hill on a curved, traffic-free road in tandem with one going the opposite way.
It goes between between Restauradores Square and the Bairro Alto neighbourhood renowned for its nightlife.
Lisbon's City Council halted operations of three other famous funicular streetcars in the city while inspections were carried out.
The Elevador da Gloria is classified as a national monument.
Carris, the company that operates the streetcar, said that scheduled maintenance had been carried out.
It offered its deepest condolences to the victims and their families, and promised all due diligence would be taken in finding the causes.
President Marcelo Rebelo de Sousa offered his condolences to affected families, and Lisbon Mayor Carlos Moedas said the city was in mourning.
"It's a tragedy of the like we've never seen," Moedas said.
Portugal's government announced that a day of national mourning would be observed on Thursday.
European Union flags at the European Parliament and European Commission in Brussels flew at half-mast.
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