The driver of the black Ford Fiesta was seen to attack the cyclist at the junction of Brooms Road and Cumberland Street at about 2.15 pm on Tuesday.
Witnesses say he got out of the car, approached the passing cyclist and then assaulted him. The 27-year-old male victim received injuries to his nose, but did not require medical attention.
Constable Lee-Ann Hawker at Dumfries said: “This appears to have been a completely unprovoked attack on a lone cyclist in the town and we at a loss to understand why.”
The driver is described as male, aged 20-25 years old, of big build with a spotty complexion and slight beard. He had short ginger hair.
PC Hawker said: “Anyone who may have been in the area of Brooms Road/Cumberland Street around this time and who can help identify him is asked to call us at Dumfries in the 101 number.
“The car is thought to be a three door, black coloured Ford Fiesta with darkened windows and was last seen travelling up Cumberland Street.”