Seine-et-Marne is a department in the Île-de-France region in Northern France. Named after the rivers Seine and Marne, it is the region's lar…Seine-et-Marne is a department in the Île-de-France region in Northern France. Named after the rivers Seine and Marne, it is the region's largest department with an area of 5,915 square kilometres ; it roughly covers its eastern half. In 2019, it had a population of 1,421,197. Its prefecture is Melun, although both Meaux and Chelles have larger populations.