The incident happened around 7.55am today (Thursday) on the A6089 Gordon to Kelso road, around half a mile North of Kelso.
A 47-year-old male cyclist collided with a southbound Mercedes coach.
Police and ambulance attended and the cyclist was taken to the Edinburgh Royal Infirmary where his condition is described as serious.
There were no passengers on the coach and the driver was uninjured during this incident.
The road was closed the road for approximately 2 hours whilst officers examined the scene.
A spokesman for Lothian and Borders Police said: “We are investigating the circumstances that resulted in this collision and would encourage any motorists who were on the road at the time to contact police immediately, particularly the occupants of 2 cars who stopped at the scene.
“Similarly, anyone with any other information that can assist police with their enquiries is also asked to come forward.”