The collision happened at 12.14pm yesterday, 10 November, between Cheesfoot Head and Cheriton, and was between a white Triumph motorcycle and grey Renault Captur.
The rider of the motorbike, a 74-year-old man from Waterlooville, was taken to Southampton General Hospital with life-threatening injuries.
The driver and passenger of the Renault also received minor injuries.
Officers are investigating the exact cause of the collision and are keen to speak with anyone who may have information.
Were you on the road at the time? Perhaps you have dash-cam footage of the vehicles prior to the collision?
Please phone 101 with reference 44200435642. Alternatively, you can call Crimestoppers 100% anonymously on 0800 555 111.