RCMP Cpl. Krista Hobday said the teen was with several people walking across the highway south of the downtown core at about 3 a.m. Monday when she was hit by a vehicle at an intersection with traffic lights.
Hobday said the driver was helping the girl when a second motorist struck her and an 18-year-old man.
The second motorist apparently did not see that the first vehicle was stopped, she said.
The girl did not survive, and the man was taken to hospital with serious injuries.
Hobday said the area is well lit and that alcohol does not appear to be a factor.
"You've got two extremely traumatized drivers. The first one felt bad enough and now the second one .... You can imagine yourself in that situation, how horrible you'd feel."
Several others who were with the two pedestrians left the scene before an ambulance arrived, Hobday said.
Police are asking anyone with information to contact them.
The coroners service is also investigating.