The attacker performed a sex act in front of the victim outside a home in Pinner View, Harrow, at around 5pm on December 3.
He then left the scene on a bicycle.
The victim was extremely distressed following the incident and is being supported by specialist officers.
Police have now released an e-fit image of a man they wish to identify and speak to in connection with the offence.
Detective Constable Oneda Mehmeti, from the Metropolitan Police’s North West Command, said: “Offences like this are extremely serious and can have a significant and lasting impact on victims.
“A number of enquiries have been carried out including forensics and speaking with witnesses. We are now hoping that the public can help us identify this man who we would like to speak to.
“I am grateful that the victim in this case came forward.
“Reporting crimes of this type is vital in order for us to identify and apprehend offenders.”
Anyone with information concerning the identity of the man pictured should call 101 or Tweet @MetCC quoting reference CAD 4664/03DEC23.
Alternatively contact the independent charity Crimestoppers anonymously on 080 555 111 or visit crimestoppers-uk.org.