Inspector Peter Duncan said: “The 19 year old female and a 21 year old male were struck by a van on the Moneynick Road at around 3:40 am.
“The female died as a result of her injuries while the male is currently in a very serious condition in hospital.
“We are appealing for anyone who may have witnessed the collision or who may have any information that could assist the police investigation to contact police by calling 101 quoting reference 176 of 20/01/18.”
The Moneynick Road remains closed in both directions near the Toome service station.
Belfast-bound traffic from Derry is being diverted down the Roguery Road.
North-bound traffic to Derry is being diverted down the Lismacloskey Road onto the Staffordstown Road.