Police are searching for creep who they believe exposed himself to a 9-year-old girl on a Melbourne tram.

Detectives have been told a young girl was with sitting opposite a family member on Tram 48 along Collins Street towards Docklands about 2.30pm last Monday afternoon.

The man positioned himself near the child and exposed himself, then committed a lewd act.

The girl told her family member who then confronted the man,  and he fled from the tram as it stopped on Collins street (between Williams and Market streets), running towards Market street to Flinders Street.

Investigators have released a facial composite and CCTV of a man they believe may be able to assist with their enquiries.