A woman was indecently assaulted on a tram in Melbourne's inner north this week.

Police say the woman was assaulted in Parkville on a city-bound route 55 tram about 8.30am on Thursday.

A man who boarded the tram had been acting disruptively before he stood behind the victim and indecently assaulted her.

A male and a female passenger stepped in to help the victim, who then got off the tram.

The offender is described as 185 centimetres tall, aged in his 30s, of medium build with short black hair and beard, and being of Maori or Pacific islander appearance.

Transit crime investigation detectives have asked for anyone with information or who witnessed the assault to phone Crime Stoppers on 1800 333 000 or visit www.crimestoppers.com.au.