Vandals damaged a third of the buses used to bring about 18,000 students to public schools, leading officials to give children an unexpected day off from school Monday.
The city's schools were closed because vandals sprayed fire extinguishers, spray-painted graffiti, smashed first aid kits, damaged fuse boxes and broke antennas on 40 buses.
A worker for the bus company discovered the damage Sunday night, police Lt. Chris Corbett said.
Safety equipment must be replaced before the buses can be used again, he said.
"Hopefully, they'll be ready to roll tomorrow morning," Corbett said.
No arrests had been made but authorities thought juveniles may be involved because the damage came shortly before Halloween.
Waterbury, which has about 120,000 residents, is in central Connecticut.
In early October, Newark, Ohio schools were closed after vandals damaged the city's school bus fleet. The vandals discharged fire extinguishers, dumped trash in the buses and stuffed paper in the gas tanks.
Prosecutors charged two fifth-grade boys who were seen on a surveillance camera at the bus lot.