The incident, which involved a Triumph GSR motorcycle and Audi TT, occurred just after 2.30pm on Thursday on the B2177 Southwick Road.
The motorcyclist, a man in his 50s from Southampton, suffered a serious back injury in the collision.
Now officers investigating the exact circumstances of the collision are appealing to anyone who witnessed the incident or who has dash-cam footage of the crash.
Anyone with information can call 101, quoting the reference 44220026216.