An East Midlands Ambulance Service crew found the bin when they tried to leave for hospital in Leicester.
Waste fluid had leaked into the cabin, forcing the vehicle to require a deep clean.
The ambulance was taken out of service and a second ambulance was called. EMAS said the action could have put lives at risk.
The crew was responding to a call in Narborough Road, Leicester, in the early hours of Monday.
Police were notified and the ambulance was returned to service the next day.
Tim Hargraves, an EMAS manager based in Leicestershire, said: "This behaviour is disgusting and can ultimately put lives at risk as this vehicle was taken off the road.
"Our crews work long hours in challenging circumstances and deserve to be treated with respect.
"We will work with Leicestershire Police to support enquiries."
A Facebook post on the EMAS page received more than 1,500 messages of support and was shared more than 4,000 times.
A wheelie bin of rubbish was dumped on the front seats of an ambulance as medics treated a patient in the back.