The incident happened around 3.50pm yesteday on the westbound carriageway, between the Straiton and Lothianburn junctions.
A Ford Transit van has collided with a Volkswagen Caddy van, forcing it into the central reservation before also colliding with a Honda motorcycle.
The 29-year-old male driver of the Transit was uninjured, while the 42-year-old male drive of the Volkswagen sustained only minor injuries.
However, the 60-year-old male motorcylist sustained various serious injuries and has been taken to the Royal Infirmary of Edinburgh where he remains in a critical condition.
Road Policing Officers responded along with other emergency services and the road was closed for around six and a half hours while an investigation was conducted at the scene.
Anyone who can assist police with this ongoing inquiry is asked to come forward.
Sergeant Fraser Wood from Edinburgh’s Road Policing Unit said: “This collision has resulted in the motorcyclist sustaining very serious injuries and we are continuing with our efforts to establish the full circumstances of the incident.
“I would ask other motorists who were on the Bypass at the time, and witnessed what happened, to contact police immediately.
“We would also be keen to obtain any dash-cam footage from vehicles that were in the area of the collision and if you believe you have any then please also get in touch.”
Those with information can contact the Road Policing Unit in Edinburgh via 101 and quote incident number 2556 of the 4th July.
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