Images show the devastation caused by the thug, who hurled a rock at the vehicle in Castle Bromwich, Birmingham.
The ambulance was using its blue lights at the time to race to help a patient struggling to breathe.
The ambulance was struck at 8.50pm last night, putting it out of action.
A second ambulance had to be despatched to treat the patient.
Today, the damaged ambulance was off-road and out of service.
Repairs will cost hundreds of pounds accoring to the ambulance service.
Ambulance bosses have appealed for members of the public to come forward to police with information.
Emergency Services Operations Delivery Director, Nathan Hudson, said: “It is incredibly frustrating to once again find ourselves questioning why on earth somebody would want to do this to an ambulance.
“Not only that, but given it was using blue lights at the time, whoever did this could clearly see it was responding to an emergency, what if it had been on the way to one of their family members or rushing a loved one to hospital?
“We’re now faced with a vehicle being out of service, a repair bill and not forgetting the horrible nature of this incident for the staff involved.
“I am extremely thankful that no-one was hurt.
“CCTV is now being retrieved from the ambulance of the incident, on Collector Road.”
Anyone with information should contact West Midlands Police on 101 and quote 20be/75740d/19.