The bridge will be built to a design led by architect Renzo Piano.
The two companies are a new company called Pergenova. The aim is to complete the structure in just 12 months from the time when the old bridge is demolished.
The original bridge collapsed in August, killing 43 people.
The bridge connects the port and the wider coastal area to France.
The new structure will have a continuous steel deck 1,100m in length with 20 spans. It will have 19 elliptical piers made of reinforced concrete, most positioned 50m from each other. Two of the piers will be 100m from each other to span the river and the railway.
Fincantieri will build the structural steelwork at its Genoa-Sestri Ponente shipyard, as well as at the Valeggio sul Mincio (Verona) facility and, if necessary, at its other yards in Italy.
Deck elements will be welded into single spans on site, with work carried out at ground level to minimise operations at height.
An integrated monitoring, control and inspection system for the bridge will be provided.In partnership with Cetena, a company within the Fincantieri Group.