Emergency Services were called the scene on the N71 at Barleyhill East, Rosscarbery at around 9am.
Gardaí have launched an investigation after the cyclists in his 60s was seriously injured in the crash.
The man was airlifted to CUH for treatment and no other injuries were reported.
The road between Rosscarbery and Leap remains closed for a technical exam by Forensic Collision Investigations and local diversions are in place.
Gardaí are appealing for any witnesses to come forward and anyone with dash cam footage for the area between 9am and 10am is asked to make the footage available to Gardaí in Clonakilty.