The collision happened around 3.45pm on Thursday 5 November on Newgate Lane East and was between a blue Suzuki motorcycle and a dark van – police believe this to possibly be a grey Renault or Vauxhall van.
The rider of the motorbike, a 57-year-old man from Holbury, was taken to Southampton General Hospital with serious leg injuries.
The suspect is described as being a white man, in his 30s and with black hair. It is likely that the vehicle sustained significant damage to the front of the vehicle.
Officers have conducted initial enquiries but are now turning to the public for help.
Were you on the road at the time? Perhaps you have dash-cam footage of the incident?
Anyone with information is asked to call 101, quoting 44200429733.