Police want to talk to this man over the train assault.

A 19-year-old woman has been indecently assaulted on a crowded Perth-to-Mandurah train.

Police say the woman boarded the train at the Perth Underground station just after 6pm on Tuesday, June 24, and went into the third of fourth carriages of the six-carriage train.

The train was crowded and most of the seats were taken.

A man who boarded the train at the same station sat next to the woman.

Police say he indecently assaulted the woman and left the train at the Bullcreek station.

The man is described as being between 20 to 30 years of age with an olive complexion, approximately 180cms tall, of slim build with short dark hair and a small patch of facial hair under his bottom lip.

He was wearing a black and grey hooded jumper and black shorts. He was carrying a soft yellow cooler bag.

Police would like to speak with a male passenger who spoke to the victim after the man left the train.

Anyone who can identify the man or has any information, or who saw the incident is asked to call Crime Stoppers on 1800 333 000.