The incident took place on Sunday 11 April at around 10.45am, police were called to St Lawrence Shute, near the junction with Seven Sisters Road, after a cyclist collided with a wall and a safety barrier.
The cyclist, a man in his 50s from Ryde, suffered serious injuries and is being treated at Southampton General Hospital. He was riding a black and red Trek road racing bike.
Officers believe there were a group of three cyclists nearby who may have witnessed the incident but left the area after the arrival of emergency services.
If you were one of these cyclists and you saw what happened, please contact officers.
Equally, anyone with information about the incident is asked to contact police on 101, quoting 44210136102.