An investigation was launched after the Metropolitan Police received reports that a 15-year-old girl had been sexually assaulted near to Parliament Hill Viewpoint.
Police say the alleged attacker was a male “aged in his late teens”.
The assault took place at 10.45pm on Saturday (June 3).
The male is described as around 5ft 8ins tall, with short brown hair. He was wearing a blue T-shirt and jeans.
Detective Sergeant Sam Lockstone, from the local policing team in Camden, said police believe there was a large group of teenagers on Parliament Hill at the time of the assault.
They added that the victim and suspect are thought to be part of different friendship groups.
DS Lockstone said: “We have carried out a number of enquiries, including a trawl for CCTV and a forensic examination of the scene.
“I am asking for anyone who witnessed suspicious activity, or has information about the incident, to contact us immediately.
“I would also encourage parents of children who were in the area last night to ask them if they can assist the investigation.”
Anyone with information is asked to call 101 quoting CAD 7852/03Jun.
To remain anonymous, contact the independent charity Crimestoppers on 0800 555 111.