A teenage boy was allegedly sexually assaulted as he got off a peak-hour train in Melbourne's west, police say.

Transit Crime Investigation Unit detectives have appealed for help to find the man who allegedly sexually assaulted the 17-year-old as he got off the train at Footscray railway station.

The man police want to speak to over the alleged sex assault at Footscray train station.

The assault occurred on February 22, about 5.20pm.

The man police wish to speak to then boarded a Sunbury-bound train shortly after and got off at Albion railway station.