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.
Most of the valley is under Vercelli Province's jurisdiction, yet three specific towns – Romagnano Sesia, Prato Sesia, and Grignasco – are actually part of Novara Province.