The 20-year-old was with three friends in Union Street, Greenock, when he was assaulted by a man with a large bladed weapon at about 3.40pm on Saturday.
Police said he sustained significant injuries and was taken to Inverclyde Royal Hospital before being transferred to Queen Elizabeth University Hospital in Glasgow, where he is said to be in a critical condition.
Detectives said the attempted murder is believed to have been “targeted” and they are understood to be following a positive line of inquiry
The victims’s friends, aged 19, 20 and 31, were not injured.
The suspect is described as white, in his 20s, with a tanned complexion, and was wearing dark clothing.
He ran off after the attack through the church and into Nelson Street.
Detective Chief Inspector Fil Capaldi said: “Although we believe this to be a targeted attack, it was shocking in its viciousness and the fact that it took place in the middle of the afternoon in a very busy thoroughfare in the centre of Greenock.
“Officers have been checking CCTV and are in the area at the moment, and we would appeal to anyone who was there yesterday or who has any information that will assist officers with their inquiries to come forward.”
Police can be contacted on 101 or information can be provided to Crimestoppers anonymously on 0800 555111.