A blue Citroen C1 was set alight in the HMP Winchester staff car park between 10 and 11 p.m. on Sunday, June 19.
The fire service arrived and extinguished the fire.
The Citroen was completely destroyed, and the fire also damaged a car parked nearby.
Nobody was hurt.
Officers looking into the incident want to speak with anyone who may have seen anything suspicious.
Were you in the vicinity at the time of the incident? Did you notice any suspicious behaviour?
Maybe you were driving in the area and have dashcam footage?
Anyone with information that could help our investigation should call 101 or report it online at https://orlo.uk/fbjw5 quoting reference 44220244337.
Alternatively, people can call Crimestoppers anonymously at 0800 555 111.