Officers say the incident happened on the A710, Dumfries to Dalbeattie road, when a 79-year-old man was found seriously injured at around 8.15 pm on Tuesday, August 25.
He was taken to Dumfries and Galloway Royal Infirmary where he later died.
Dumfries and Galloway Road Policing Sergeant Bob McNay said: “Our thoughts go out to the family and friends affected by this incident.
“Enquiries continue and we are appealing to anyone who can assist us in establishing the full circumstances surrounding this.
“We are keen to speak to the driver of a dark coloured BMW that was seen in the area shortly before the cyclist was found.
“I would ask that if you noticed a cyclist on this road or have any information that could help with our enquiries that you get in touch with police.”
Officers have asked if motorists with any possible dashcam footage of the area to come forward.
Police can be contacted on 101 quoting incident number 3297 of Tuesday, 25 August, 2020.