Cossonay is a municipality in the canton of Vaud in Switzerland. It is the seat of the district of Morges. Cossonay has both Roman ruins and medieval graves. The first documentation of the settlement dates from 1096 under the name Cochoniacum. In 1164, this appears as Cosonai, and in 1228 as Cossonai. Cossonay received city rights in 1264. A fire near the end of the 14th century destroyed much of the town and the town archives. In 1421, the town passed to the house of Savoy.