Officers were called at around 1.35am on Friday, July 25, to reports of a woman in distress outside in Brampton Road, Hillingdon.
The woman, in her 30s, told officers she had been walking in Charnwood Road, Hillingdon, which connects with Long Lane, at around 1am when she was approached from behind by a man who pushed her to the ground and raped her, before making off from the scene.
The victim did not require hospital treatment and was referred to the Haven for support.
Officers from Sapphire, part of the Sexual Offences, Exploitation and Child Abuse Command, are investigating.
Anyone who witnessed the incident or who has information which could assist the investigation is asked to call officers on 101 quoting ref: 0915019/14.