The 16-year-old was waiting at a bus stop at Chichester Street, between the junctions of Callender Street and Arthur Street, when an unknown man sexually assaulted her.
The incident happened sometime between 12.05 pm and 12.15 pm on Friday, September 23.
Police said as the girl boarded the bus it is believed that this male followed her.
A PSNI spokesperson added: “The male is described as being aged in his mid-to-late 40s with short dark hair that was shaved on the top, black stubble, thin square face with a tanned complexion.
“He spoke with a foreign accent and was wearing a burgundy or purple waist length coat.
“Police would like to hear from an elderly male with grey hair and beard, who let the female go ahead of him in the queue in order to get on the bus, and then sat near her as he may have information which would assist our enquiries.”
Anyone with information can pass it to the PSNI on the 101 non emergency number.