The collision happened at the junction by the Le Grand Courtil traffic lights at around 18:15 on Tuesday evening.
A spokesperson for the emergency services said the cyclist suffered “multiple injuries” and has been taken to the Emergency Department.
Guernsey Police and St John Emergency Ambulance attended the scene to treat the patient. Police have now put out an appeal for witnesses, saying: “we would particularly like to speak to anyone who was travelling along La Route du Cornus heading into St Martin’s and witnessed the collision.”
Anyone with information is asked to contact Guernsey Police on 222 222, or contact CrimeStoppers anonymously on 0800 555 111.
The incident was one of 21 call outs in 24 hours for the Emergency Ambulance Service.
Earlier in the day, the Flying Christine III was sent to Sark for a patient with an injury requiring hospital treatment.