The girl was walking through the South Walk alleyway between Stratton Way and Rowan Crescent in Biggleswade when the assault occurred on Saturday.
She was walking through the alleyway between 7pm and 9pm when she was approached by a man in black clothing.
He pushed her against a wall and sexually assaulted her before running off.
The man is described as white, about 5 ft 11 in tall with brown hair and brown eyes.
He was wearing black clothing, with his face half-covered and black trainers with red and white detailing.
Detectives have done some initial house-to-house and forensic enquiries but they would like to hear from anyone who was in the area at the time.
Det Con Debbie Jones, who is leading the investigation said: “This girl has been terribly distressed by what happened and understandably so.”
Anyone with information can contact Det Con Jones on 01582 473420.