Police say the suspect allegedly followed the girl onto the #130 Metrotown bus at Hastings and Willingdon at about 3:45 p.m. PT Aug. 31.
The suspect allegedly pressed up behind the girl, pinning her and grabbing her buttocks, as she stood near the rear door of the bus.
The girl got off the bus and ran to a nearby SkyTrain station for help.
The suspect is described as a bald black man between 25 and 35 years old and about five feet ten inches tall.
He was last seen wearing a black long-sleeved shirt with a white stripe on the sleeves and a small red label on the right side of the chest.
Anyone with any information is asked to call Transit Police at 604-515-8300 or Crime Stoppers at 1-800-222-8477.