Lys was a department of the First French Empire in present-day Belgium. It was named after the river Lys. It was created on 1 October 1795, when the Southern Netherlands were annexed by France. Before its occupation, its territory was part of the county of Flanders. Its capital was Bruges.
- Bruges, cantons: Ardooie, Bruges (5 cantons), Gistel, Ostend, Ruiselede, Tielt and Torhout (2 cantons).
- Kortrijk, cantons: Avelgem, Harelbeke, Ingelmunster, Kortrijk (4 cantons), Menen, Meulebeke, Moorsele, Oostrozebeke and Roeselare.
- Veurne, cantons: Diksmuide, Haringe, Nieuwpoort and Veurne.
- Ypres, cantons: Elverdinge, Hooglede, Mesen, Passendale, Poperinge, Wervik and Ypres (2 cantons).
Its population in 1812 was 491,143, and its area was 366,911 hectares.
After Napoleon was defeated in 1814, the department became part of the United Kingdom of the Netherlands. Its territory corresponded more or less with the present-day Belgian province of West Flanders.