The man was walking along Onslow Road, near the railway bridge at Drumry Railway Station, at about 05:30 on New Year's Day when he was attacked by a group of people.
He was taken to the Queen Elizabeth University Hospital where he is being treated in the high-dependency unit.
Police said there was no apparent motive.
Officers have been reviewing CCTV from the area and discovered that there were a number of people in the area at the time.
Det Insp Grant MacLeod said: "This appears to be a totally random attack, with no apparent motive, leaving a man seriously injured.
"Our inquires have revealed that despite it being fairly early in the morning, there were various people in the vicinity at the time of the attack and I am appealing to these people to get in touch.
"It's possible you can provide information on the suspects which would assist our investigation in locating those responsible for this violent attack."
A 40-year-old man has serious injuries after being stabbed by a gang in a "random" attack in Clydebank.