Emergency services were called to Moonee Ponds Creek Trail in Glenroy, about 14km north from Melbourne's CBD, on Sunday afternoon after a passerby found the man moaning and unconscious.
The man was treated by ambulance paramedics at the scene before being taken to hospital to be treated for critical injuries.
Police said the man, who is believed to be aged in his early 20s, did not have any belongings or identity documents with him and nothing of note was found at the scene.
"Ongoing enquiries by police throughout the evening and overnight – which included checking missing person reports and an examination of his fingerprints and the scene - has failed to identify the young man," Victoria Police said in a statement.
He is described as Caucasian in appearance, about 175cm tall, has a slim build, hazel-coloured eyes with no facial hair and brown, shoulder-length curly hair.
He was wearing a grey singlet top and ripped blue jorts, a style of baggy denim shorts, when he was found.
He also has residue of dark red nail polish on some fingernails.
At this stage, police do not believe the man was assaulted, but it is unclear how he became injured.
One possibility police are exploring is that he may have fallen or slipped from the Moonee Ponds Creek Viaduct, which sits above Kingsford Smith Ulm Reserve, where the man was found.
Due to the seriousness of his injuries, police are urgently trying to identify the young man to ensure his family are aware of his condition.
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