The ambulance's windscreen was cracked when the object was thrown in Handsworth Wood at around 8.20am on Saturday.
The crew, made up of a paramedic and two student paramedics, were forced to pull over due to the damage and a second ambulance had to be sent to take the patient to hospital - delaying the response by around 10 minutes.
Police officers were sent to the scene on Worlds End Road and CCTV footage captured by the ambulance will now be examined. So far no arrests have been made.
None of the crew were injured.
Nathan Hudson, emergency services operations delivery director at West Midlands Ambulance Service, said: “It is difficult to comprehend why someone would do something like this.
“There is only one thing that an ambulance travelling on blue lights is doing and that is trying to get to a patient in need.
“Given where the damage is, I am just so glad that none of the crew were hurt. It doesn’t bear thinking about, what could have happened.
“We will work with the police to do everything possible to find the individual concerned so that they can be out before the courts.”
The ambulance involved is based out of the Erdington hub.
A West Midlands Police spokesman said enquiries were ongoing. Anyone with information is asked to contact the force quoting crime reference number 20/1761687/21.