Graffiti referencing the Ibrox disaster and former player Ian Durrant was scratched into the paintwork of the coach in the early hours of this morning.
The sides are playing each other in a league clash at Pittodrie.
Officers said their inquiries were at an early stage and appealed for information.
A spokesman for Rangers FC said: “The Club is disgusted and disappointed by this act of vandalism. In particular, the reference to the Ibrox Disaster in which 66 people lost their lives is especially vile.
“This sort of behaviour has no place in civilised society.
“The incident was immediately reported to Police Scotland and we will assist them with their enquiries.”
An Aberdeen spokesperson said: “We are particularly disappointed with this act of vandalism and do not condone it.”