The incident left a Gourock man with serious head injuries and he is now in Glasgow's Southern General Hospital undergoing emergency surgery this morning.
Police say he was assaulted by another male at Cardwell Road in Gourock at 12.30am.
Detective Sergeant Gerard McWatt, of Greenock CID, said the police investigation is in its early stages and he has issued a public appeal for information.
DS McWatt said: "We are treating this incident as attempted murder.
"I would ask any witnesses or anyone with information to contact police on 492500."
Cardwell Road was cordoned off with police tape this morning and uniformed officers remained on scene.
Police were also seen speaking to residents for information as investigations continue.