Sésia was the name of a département of the First French Empire. Situated in present-day Italy, it was named after the Sesia river. Its capital was at Vercelli.
The département was formed on 11 September 1802, when Napoleon Bonaparte occupied the Kingdom of Sardinia. The département was subdivided into the following arrondissements and cantons (situation in 1812):
- Vercelli, cantons: Agnona, Crevacuore, Gattinara, Masserano, Quinto, Stroppiana, Trino and Vercelli (2 cantons).
- Biella, cantons: Biella, Bioglio, Cacciorna, Candelo, Cavaglià, Cossato, Graglia, Mongrando and Mosso Santa Maria.
- Santhià, cantons: Buronzo, Cigliano, Crescentino, Livorno and Santhià.
Its population in 1812 was 202,733, and its area was 335,118 hectares.
It was disbanded after the defeat of Napoleon in 1814. Its territory is today divided between the Italian provinces of Vercelli and Biella.