Officers were called to the westbound carriageway between junctions 15 and 16 around 22.50 GMT on Saturday, 5 November 2022.
A 51-year-old man was taken by ambulance to the Queen Elizabeth University Hospital with serious but not life-threatening injuries.
The road was closed while investigations were carried out and reopened around 01.30 on Sunday, 6 November 2022.
Enquiries are ongoing into the circumstances and officers are appealing for witnesses.
Sergeant Chris Hoggans, Road Policing Unit, said: “I would urge anyone who was on that stretch of the M8 around 10.50pm on Saturday and saw a man on the carriageway to get in touch with police. It’s important we find out exactly what happened.
“Similarly, if you have dash cam footage from the area at the time, please review it and bring anything of significance to our attention.”
Anyone with information should contact Police Scotland on 101 quoting reference 4677 of 5 November 2022.
