A man has indecently assaulted a woman by watching pornography on his smartphone on a Melbourne train.

A 20-year old Reservoir woman was sitting against the window of a carriage on a city-bound South Morang train at Bell Railway Station in Preston, just after 3.15pm, on July 7, when an unknown man approached her, Transit Crime Investigation Unit detectives said.

Police are hunting for a man who watched pornography on a Melbourne train. Credit:Victoria Police

Police said he hovered over her for a short time before sitting next to the woman and leaning against her with his elbow pressing against her arm.

He then placed his mobile phone on his lap with the screen facing upwards and began to play pornography for about five minutes.