The incident happened between 6.20pm and 6.30pm on Friday, January 26.
Both vehicles, a Honda motorcycle and an Infiniti EX30D, were travelling on Cakers Lane when they collided with each other.
The rider of the motorcycle, a 21-year-old man from Bordon, suffered serious injuries and is being treated at Southampton General Hospital.
PC Martin Norman said: “We’re keen to speak to anyone who witnessed the collision or saw either vehicle prior to the incident.
“We know there is a potential key witness who was driving along Cakers Lane towards Alton at the time of the collision. We would urge this person, and anyone else who saw what happened, to contact us as soon as possible.”
Anyone with information should contact Hampshire Constabulary on 101 quoting 44180034728.