Police are appealing for public assistance to identify a man who died in hospital after being hit by a truck in Braybrook, northwest of Melbourne.

Police believe the man stepped out from the footpath and was struck by an Isuzu truck on Ballarat Road about 6pm (AEDT) on Wednesday.

The man was taken to the Royal Melbourne Hospital with life-threatening injuries, but died about 9.50pm that night.

Investigators have been unable to identify the man and no missing person reports have been filed which fit his description.

Photographs of three distinctive tattoos are being released in the hope that family or friends of the man come forward and identify him.

Anyone who may have witnessed the accident, or who is able to identify the tattoos depicted is urged to contact Crime Stoppers.

AAP