The bridge, built in the 1960s, stands on the A10 toll motorway and is known as the Morandi bridge. The missing section was dozens of metres in length, and ran across the span of the Polcevera stream.
Italian newspaper La Repubblica described that part of the city as “densely inhabited”.
The structure collapsed shortly before noon local time (10:00 GMT) during heavy rain.
An eyewitness told Italian public television there was a queue of traffic on the bridge at the time. Initial reports indicate there may be people trapped in the rubble.
Pics via Antonello Guerrara