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.
Part of the viaduct was washed away by a mudslide.
Vigili del Fuoco/AFP via Getty I
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.