Two vehicles collided on the A32 Gosport Road shortly after 08:00 GMT, Hampshire Constabulary said.
The biker was treated by paramedics at the scene but later died from his injuries.
Motorists are urged to avoid the area as the road, which has been closed near Wych Lane, will remain shut for several hours.
A motorcyclist has died after being involved in a crash with a lorry in Fareham, Hampshire.