Officers have appealed for witnesses following the attack, which is reported to have taken place close to Hounslow bus garage on Wednesday May 6.
The woman, who is aged in her 40s, was walking along Kingsley Road, near the junction with Cecil Road, at around 9.40pm when she was approached by four or five men.
One of the men allegedly slapped and sexually assaulted her and there was a short verbal exchange between the two as the rest of the group closed in, police said.
The main suspect then made further physical contact with the victim before walking away with the rest of the gang.
All the men are described as being of Asian appearance.
The main suspect is believed to be aged 25-29, approximately 5ft 9ins tall and was wearing dark trousers and a white T-shirt.
Detective Sergeant Jason McElroy said: "We are keen to trace one witness in particular who is described as an Asian man who had been in close proximity to the victim at the time of the incident.
"This was clearly a distressing incident for this victim who was intimidated and assaulted along a route which she routinely walks."
No one has been arrested in connection with the incident.
Anyone with information is asked to call police on 020 8247 6273 or Crimestoppers, free and anonymously, on 0800 555 111.