Lagny-sur-Marne is a commune in the eastern suburbs of Paris, France. It is located in the Seine-et-Marne department in the Île-de-France 26.1 km from the centre of Paris.
The commune of Lagny-sur-Marne is part of the Val de Bussy sector, one of the four sectors in the "new town" of Marne-la-Vallée.