Essonne is a department of France in the southern Île-de-France region. It is named after the river Essonne. In 2019, it had a population of 1,301,659 across 194 communes. Essonne was formed on 1 January 1968 when Seine-et-Oise was split into smaller departments. Its prefecture is Évry-Courcouronnes. Its INSEE and postcode number is 91.