The 48-year-old man was found seriously injured on Union Street having been assaulted.
The incident was reported to police shortly before 11.30am.
He was taken to the Queen Elizabeth University Hospital with serious injuries.
Detective Sergeant Chris Curness said: “We are appealing for any witnesses and anyone with information to get in touch with officers to assist with our enquiries.
“Anyone with information should contact police on 101 quoting incident number 1210 of 30 June or call Crimestoppers on 0800 555 111.”