The incident took place shortly after 3pm on Tuesday (June 21) as the girl was walking the path leading from Stradbroke Road to Coisley Hill.
It was here a man reportedly approached her, grabbed her by the throat and sexually assaulted her.
The man is described as white, around 5ft 6 tall, of a muscular build with red blotches on his face, blue/grey eyes and greying stubble. He was wearing a black, long-sleeved jumper, black or grey trousers or jeans, dark coloured shoes or boots, a black beanie hat with a fold around the rim, and a black face covering.
Detective Inspector Chris Ronayne, leading the investigation, said: “The victim of this attack has shown great bravery in reporting this to us and she continues to be supported by her family and specialist services.
“We want to hear from anyone who was in that area at the time of the attack and may have noticed something suspicious to please get in touch with us. Did you see someone loitering in the area or acting suspiciously?
“We are particularly keen to speak to two school girls who the victim saw as she walked along the path. Please come forward and speak to us as you could help us with our investigation."
Anyone with any information which could help the inquiry is asked to call 101 quoting incident number 700 of June 21.