The driver was among those killed and 15 people were injured.
Italy has launched an investigation after a fully electric-powered bus with foreign tourists veered off a busy overpass on the Venetian mainland during rush hour.
21 people were killed.
The bus with 35 tourists onboard crashed through a guardrail and a rusted fence on Tuesday night and plunged more than 10 meters (30 feet) to land upside down.
The driver was among those killed and 15 people were injured.
The bus was just a year old.
The 40-year-old driver had an untarnished record and had just started his shift.
Venice's mayor called the crash 'inexplicable' while the regional governor suggested the driver may have fallen ill.