The collision happened on Hamble Lane, Bursledon at around 5.30am.
As a result of the crash the road has been closed in both directions and is expected to remain shut for several more hours.
The cyclist has been taken to University Hospital Southampton following the serious crash this morning.
A spokesman for Hampshire Constabulary said: ‘We were called at 5.40am today to a report of a serious collision involving a bicycle and a van.
‘The cyclist suffered serious head injuries. Hamble Lane is currently closed in both directions while we attend the scene.
‘Officers at the scene have advised people to avoid the area for any non-emergency travel.’
A spokesman for South Central Ambulance Service added: ‘We received a 999 call at 05:33 with the caller reporting a serious collision between a vehicle and a cyclist on Hamble Lane, Bursledon.
‘We sent two ambulances, a paramedic team leader in a rapid response vehicle and an ambulance officer in a rapid response vehicle to the scene.
‘The rider was a male, estimated to be in his 50s, who had sustained serious and potentially life-threatening injuries. Following initial treatment at the scene, he was then taken to University Hospital Southampton.’
Hants Road Police tweeted: ‘Hamble Lane is closed both directions and will be for some time. Please do NOT attempt to use. Please watch this feed for updates.
‘All non emergency travel using Hamble Lane advised not to attempt to travel there at the moment. This will leave roads clear for essential users and 999 services. If everyone travels we will all be stuck. Please help us to help you.’
They then added: ‘Hamble Lane update. We hope to have the road reopen in a couple of hours. All specialists working hard to secure evidence and as soon as we can we will update you. Thank you for your help in not congesting this area.’