The incident happened at around 19:30hrs on Tuesday, 1 August as the route 283 bus drove from Hammersmith Broadway to Shepherds Bush Green.
The victim, a 13-year-old girl, boarded the bus near Shepherds Bush Green and made her way to get a seat at the rear of the bus.
The suspect, a woman described as aged in her late teens or early 20s, was sitting blocking the aisle and refused to move her legs.
She then took a drink from the girl and poured it over her before launched a violent assault. A man tried to intervene but was also assaulted by the suspect.
The bus stopped outside the Bush Theatre on Uxbridge Road, W12 and the suspect got off leaving the victim on the floor.
The victim was taken to hospital for treatment before being discharged.
PC Tom Jasper from the Met’s Roads and Transport Policing Command said:
“This was an unprovoked assault which resulted in excessive violence being used against a young girl. We have issued an image of the woman we want to identify – if you recognise this person, or have any information about this incident, then please get in contact immediately.”
Anyone who can help is asked to call 101 or ‘X’ @MetCC and quote CAD6796/1Aug. You can also provide information anonymously to the independent charity Crimestoppers on 0800 555 111.