At around 19:30pm on Tuesday the 1st of August, the 13 year old boarded the 283 bus and made her way to the rear to take a seat.
The suspect, in her 20s, blocked the isle, refused to move, then poured the girls drink over her before launching a violent assault.
A man tried to intervene but was also assaulted by the suspect and the victim was taken to hospital for treatment.
Anyone with information or who witnessed the incident is urged to contact police on 101.
