The yellow mounted device on West Wycombe Road in High Wycombe was set on fire at about 19:15 BST on Saturday.
Fire crews and the police were called out, but the fire was out by the time they got there.
Thames Valley Police said they had carried out house-to-house inquiries and were seeking information about the crime.
A speed camera fire in a residential street in Buckinghamshire is being treated as arson, police said.