The accident happened at Killyleagh Road in Downpatrick on Sunday 20th October.
The collision occurred just before 2 pm on Sunday afternoon and involved a black Mercedes car.
Both cyclists were taken to hospital following the collision and sadly the man was pronounced dead on Tuesday.
The second cyclist remains in hospital receiving treatment for injuries which are not believed to be life threatening.
Police are appealing to anyone who witnessed the collision, or who may have captured dash cam footage, to contact officers on 101, quoting reference number 984 of 20/10/19.