The man, whose age is as yet unknown, was cycling on Ludborough Road, North Thoresby, when the bicycle was in collision with a Mercedes E Class-type car at about 3.10pm.
Police wish to speak to a woman who tended to the injured cyclist but left the scene of the accident before police arrived.
Inspector Gordon Richards of Lincolnshire Police said: “As a result of the collision, the pedal cyclist has been seriously injured.
“We are aware of a woman who went to the imeediate assistance of the injured cyclist and if that person, in particular would make contact with us, things will be dealt with in the strictest confidence.”
Anyone with information should call 101, quoting incident number 239 of July 8.