Police are searching for a man who exposed himself and masturbated in front of two women at a public library in Melbourne's north.

The man walked into the Lalor library on May Road about 1.30pm on December 1.

An image of the man from CCTV footage. Credit:Victoria Police.

He was in the library for about an hour before he sat down in front of two women and exposed himself.

It is understood he began masturbating in front of the women with his genitals exposed.

He left the library once the women alerted staff.

The man was wearing a fluoro green T-shirt with pink and black writing on it, jeans and thongs at the time. He is described as being about 180 centimetres tall with a slim build and short, dark hair.

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