"It is very serious that two Norwegians have been given a death sentence," said Jonas Gahr Stoere, Norway's foreign minister.
"Norway will use all means and channels available to prevent these Norwegians being killed."
"The court declares that all crimes are established in fact and in law ... and sentences Tjostolv Moland to the death penalty ... and Joshua French to the death penalty," an unnamed officer told a packed courthouse.
The court also ruled that Norway and the two men will have to pay $60m to the DRC, on top of tens of thousands of dollars in compensation to members of the driver's family.
The ruling was met with cheers in the courthouse.
Moland, 28, and French, 27, have said they are innocent and no longer linked to Norway's military, contrary to a statement made by the court.
The Norwegians had previously served in Norway's armed forces but Oslo said their official links with the military were cut in 2007.
Human rights violations are rampant in the DRC and the judiciary is widely accused of corruption and inefficiency, but criminals sentenced to death are seldom executed by the state.
The trial has caught the attention of the media and fuelled speculation over what the men were doing in a part of the country that has seen years of conflict, but is starting to entice investors looking to tap into untouched oil reserves.
Captain Claude Disimo, the chairman of the military court in Kisangani, had earlier dismissed the defence's arguments, saying that Moland had fired the shot that had killed the driver and the two were also spies and a threat to the country's security.
"[They] are active military officers as proved by their military cards, valid until 2016 and 2017," he said.
"In that respect, they are de facto intelligence agents for their country."
Stoere said: "These are people who have not acted on behalf of Norway and who are not tied to Norwegian authorities.
"We are not in a position to consider the question of [the Norwegians'] guilt, there has been no Norwegian investigation into this case.
"We are holding Congolese authorities responsible for their safety."
Ex-soldiers are frequently taken on by private security companies active in the east of the DRC.