A missile thought to be a stone was thrown at the tram between the Highbury Vale and Bulwell stops shortly before 7pm on Tuesday.
There was a loud bang as a large window shattered but special safety glass meant that shards of glass did not fly off.
The tram continued its journey to the Hucknall stop but was then withdrawn from service.
A spokesman for Nottingham Express Transit, said: “Despite the safety glass we have fitted on the windows, such behaviour as this incident causes significant distress both to customers and staff, and it is totally unacceptable.”
Police are investigating.