The 16-year-old girl was walking alone out of Centenary Park at Bentleigh East at about 6:00pm when a man attacked her from behind.
Detective Senior Constable Donna MacDonald said police were appealing for any witnesses to contact them.
“She was grabbed from behind by a male and a short struggle occurred and the victim managed to break free,” she said.
“While man had hold of her, he sexually assaulted her. The victim has then run off towards Bignall Road.”
Detective Senior Constable MacDonald said it was a terrifying ordeal for the young girl.
“It is quite brazen, it is concerning,” she said.
“We would appeal to any witnesses in the area or saw anything, or know anything, to contact Crime Stoppers.”
The girl did not get a good look at her attacker.
Police believe it was an isolated incident, but said people in the area should be aware of their surroundings.
“Any attack on a female or young girl is concerning,” Detective Senior Constable MacDonald said. “We just suggest that the public be aware of their surroundings.”