A Ringwood man in his 50s was taken to Southampton General Hospital with life-threatening injuries following a collision on Verwood Road.
The cyclist and a silver Renault Clio collided on the B3081 near the Alderholt turning on Monday evening around 8.30pm and the driver of the car, a man in his 30s from Hampshire, was uninjured.
He was arrested at the scene on suspicion of driving offences and is currently assisting police with their enquiries.
Officers investigating are appealing for witnesses or anyone with relevant dashcam footage to come forward.
Air Ambulance attended the incident along with the Police, and the road was closed to allow emergency services to deal with the incident and for a detailed examination of the scene to take place.
The road has now reopened.
Police Sergeant Lee Savage, of the traffic unit, said: “We are carrying out enquiries to establish the full circumstances of this collision and I am appealing for any witnesses to please come forward.
“I would urge any motorists who were travelling in the area and have dashcam fitted to their vehicle to please view it for any relevant footage that may assist my investigation.
“Finally, I would like to thank motorists for their patience during the road closure, which was absolutely necessary to enable emergency services to attend the scene in a safe manner and provide medical treatment.”
Anyone with information is asked to contact Dorset Police via email at email@example.com or by calling 101, quoting incident number 29:737.