It happened just after 1 p.m. on Sunday, Nov. 29 on the No. 28 bus at Douglas and Fort streets.
Police said a man followed the girl onto the bus, where he exposed himself to her while committing an indecent act.
The man then sexually assaulted the girl, before she was able to get off the bus at Shelbourne Street and Cedar Hill Cross Road, police said.
The suspect is described as Black, between the ages of 20 and 30, about five-foot-eight and slim.
He was wearing ivory-coloured pants.
Anyone with information is asked to contact Victoria police or Greater Victoria Crime Stoppers if they wish to remain anonymous.