The assault took place at around midnight on Wednesday, June 30 in a wooded area in Clapham Common.
It is believed that two men were present at the time of the assault which was reported to police in the past week.
It was dark and no other information about the men is available, the Metropolitan Police said.
Detective Sergeant Ross Burrell, said: “We are determined to identify the person or persons responsible for this horrific crime and seek justice for the victim.
“A significant period of time has passed since the attack occurred but we still think there will be people out there with information that could help.
“We want to hear from anyone who was out on the Common that night and saw or heard anything suspicious. We want to hear from anyone who has seen two men in the area, either before or after this incident, acting suspiciously or doing anything which has raised concerns.
“It does not matter why you were in the area. You can be assured that officers will make no judgements and will treat any information you provide sensitively. Your information, no matter how small, could be vital. Please make that call.”
Anyone with information should call 101, providing the crime reference Cad 7542/17Aug.