Police say it happened as the 13-year-old was being evaluated at the Coventry Fire Station.
Officials say the keys were in the ignition and the boy got into the ambulance and drove off.
He eventually crashed into a motorcyclist, and ended up in a embankment.
The teen, the motorcyclist, and a firefighter were transported to the hospital in stable condition.
Police said the ambulance sustained only minor damage from the crash.