The alleged incident took place at about 4pm on Thursday, 12 August on the 282 bus near the White Harte Roundabout in Northolt.
A man started talking to the boy at the bus stop, sitting next to him on the upper deck of the bus after they both got on board.
On the journey, the man is said to have made remarks of a sexual nature and touched the victim inappropriately.
There have so far been no arrests as enquiries continue, but police today released an image of a man they wish to talk to.
Detective Inspector John Millward from the Met’s Roads and Transport Policing Command, said: “This was a frightening experience for the young victim. Fortunately, he did not sustain any physical injuries, but he was understandably left very shaken.
We're investigating the sexual assault of a teenage boy on a bus in #Ealing and we want to identify this man.
If you know who he is, please call 101, providing the reference 5822/13AUG or contact Crimestoppers.https://t.co/00q5Xf7uOH
— Ealing Police (@ealingMPS) October 9, 2021
“We have conducted a number of enquiries to identify the man in the photo we are releasing today and that work is ongoing. We think the public may be able to provide information that could be crucial.
“If anyone recognises him, I would urge them to come forward either directly to the police or to the independent charity Crimestoppers.”
Anyone with information should call 101, providing the reference 5822/13AUG.
Information can also be provided to Crimestoppers, anonymously, on 0800 555 111..