The man and a friend were walking a black whippet on Kilmacolm Road on Sunday night when they were set upon.
The 45-year-old man was hit with a weapon - thought to be a blade or hammer  - and needed stitches for three cuts to his head.
He was taken to Inverclyde Royal Hospital where he received treatment for his injuries.
The 48-year-old man with him was also assaulted but escaped injury.
Det Insp David Wagstaff, of Police Scotland, said the men were attacked by a group of men and women they spoke to at a bus stop on Kilmacolm Road at about 22:40.
He said: "The two men were attacked for some unknown reason.
"We would like to hear from anyone who saw the attacks or knows anything about them."
A dog walker has been taken to hospital after being attacked by a group of men and women in Greenock.