Police are searching for a man who allegedly committed an indecent act in front of a 12-year-old girl while both were sitting at the back of a bus.

A police spokeswoman said the girl got on the bus in the city on Monday, December 12, at about 11.15am and made her way to the back seat.

Police would like to speak to this man. Credit:WA Police.

"During the journey the man committed an indecent act in front of the girl," she said.

"She attempted to tell the bus driver before getting off the bus and reporting the incident."

The man is described as being dark skinned, 30 to 40 years old and approximately 180 centimetres tall with medium build.

Anyone with information is asked to call Crime Stoppers on 1800 333 000.