Police are appealing for information after the accident in Liverpool Gardens, Worthing, at 11.37pm on Friday (June 6).
A 39-year-old local man riding a blue pedal cycle was in collision with a white Mercedes-Benz Vito taxi.
The cyclist suffered serious head injuries and was taken by ambulance to Worthing Hospital, where he is in intensive care.
The 59-year-old taxi driver and his four passengers were unhurt.
Anyone who may have seen the collision or noticed either party shortly beforehand is asked to phone Sussex Police on 101 or email firstname.lastname@example.org, quoting serial 1699 of 6/6.