Police hunt a man who masturbated in front of a 15-year-old girl on Melbourne tram.

Police are hunting a man who masturbated in front of a 15-year-old girl on a tram in Melbourne last month.

The teenager told investigators she boarded a tram at Clarendon St in Southbank on June 19 at 7.15pm, when a middle-aged man began touching himself while staring at her.

The man was sitting only about a metre away from the girl.

She moved to other the end of the tram when she became uncomfortable but police say the man continued to watch her.

The victim got off the tram at Acland St, St Kilda and reported the incident to police.

Investigators are now searching for the man, believed to be in his mid-50s, of a tall and solid build, with olive skin and unshaven.

At the time of the incident he was wearing a blue and orange beanie, a black coat, a tan and black striped hooded jumper, track pants and was carrying a tan enviro-bag.

Detectives from the Transit Crime Investigation Unit have released CCTV image of a man they believe can assist them with the enquiry.

Anyone with information about the incident is urged to contact Crime Stoppers on 1800 333 000 or visit www.crimestoppersvic.com.au.