Lagny-sur-Marne (pronounced: [laɲi syʁ maʁn]; English: Lagny on the 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 (16.2 mi) from the center 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.
During the Middle Ages Lagny-sur-Marne was a popular place for tourneys in Northern France. In November 1179 a notable tournament was held by Louis VII of France in honour of the coronation of his son. Later in 1846, Lagny-sur-Marne annexed the commune of Saint-Denis-du-Port.
Inhabitants are called Latignaciens or Laniaques.
When Titus Interactive was active, its head office was in Lagny-sur-Marne.
Lagny-sur-Marne is served by Lagny – Thorigny station on the Transilien Paris – Est suburban rail line. This station, although administratively located on the territory of the neighboring commune of Thorigny-sur-Marne, lies immediately across the Marne River from the town center of Lagny-sur-marne.