Police said the girl was walking in the town's Beechwood Road shortly before midnight on Saturday when she was approached by two men.
After a short time, one of the men left. The other man then subjected the girl to a serious sexual assault.
She made her way home and police were called. CCTV footage is being examined and door-to-door inquires are taking place.
The suspect was described as being white, in his mid to late teens, possibly with blonde hair which was spiky at the front, and wearing a black hooded top. He did not have a Glaswegian accent but was Scottish.
The man who was with the suspect was also white and in his mid to late teens. He was wearing a dark coloured beanie style hat and dark coloured T-shirt.
A forensic search of the area has been carried out and officers are still reviewing CCTV images as part of their inquiries.
Det Insp Kevin Jamieson, who is heading the investigation, said: "A young woman has been left traumatised by this attack and extensive inquiries have been carried out.
"We are still in the process of reviewing CCTV images and conducting door-to-door inquiries.
"I am very keen to trace the man who was with the suspect before the attack. I know he may be unaware as to what happened after he left, but I believe he will be able to provide important information which could enable officers to trace the suspect.  I would urge him to come forward.
"I would also ask anyone who was in the area of Beechwood Road, Cumbernauld, late on Saturday evening and perhaps remembers seeing the girl or the two men walking on that stretch of road to contact police.
"Equally, I would ask members of the public who recognise the description of the suspect or have any information whatsoever, to telephone detectives at Coatbridge Police Office on the non-emergency number 101.  Alternatively please call Crimestoppers on 0800 555 111 where anonymity can be maintained."
A 15-year-old girl has been raped as she walked home in Cumbernauld.