Police are investigating after a man was sexually assaulted in a public toilet block in the city's west.

Investigators have been told a man was approached by an unknown male inside a toilet block at Footscray Park on Maribyrnong Boulevard, Footscray, about 9.30pm on September 29 last year.

The two men spoke briefly before the victim was sexually assaulted, police spokeswoman Belinda Batty said.

The victim reported the assault to a nearby police station, but a patrol of the area found no trace of the offender.

Investigators have released a facial composite of a man they believe may be able to assist with their enquiries.

Anyone with any information is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential report online at crimestoppersvic.com.au