While the valley mainly belongs to the province of Vercelli three of its comuni, namely Romagnano Sesia, Prato Sesia and Grignasco, are part of the province of Novara.
The main area of the valley is part of Vercelli Province, but a few municipalities, such as Romagnano Sesia, Prato Sesia, and Grignasco, belong to Novara Province.