The bridge collapsed on Sunday evening near the coastal city of Savona, as heavy rains have been causing widespread flooding in northern Italy this month.
Liguria's president Giovanni Toti said bad weather had caused a massive landslide that hit the viaduct.
He said one car may have been involved but that hadn't been confirmed, according to Italy's ANSA press agency.
Italy's motorway operator Sias said there have been no reports of casualties yet.
Liguria is the same region where a motorway bridge collapsed last year in Genoa, killing 43 people.
Known as Morandi bridge, it was operated by Autostrade per l'Italia, a unit of infrastructure group Atlantia.
The Sias spokesman said the toll road operator was carrying investigations to assess the situation.
He said the 50-meter long viaduct on the highway linking the coast to Turin was just outside Savona.
Motorway traffic was stopped in both directions, though only one of the two separate carriageways collapsed.
