Lagny-sur-Marne (French pronunciation: [laɲi syʁ maʁn] ⓘ, literally Lagny on Marne) is a commune in the eastern suburbs of Paris, France.

From 644, Lagny-sur-Marne was the site of Lagny Abbey, a monastery founded that year, and after its destruction by the Normans refounded about 990.

In November 1179 a notable tournament was held by Louis VII of France in honour of the coronation of his son.

[6] Lagny-sur-Marne is served by the Lagny – Thorigny station, on the Transilien Paris – Est suburban rail line.

The station, although administratively located in neighbouring commune of Thorigny-sur-Marne, is immediately across the river Marne from the centre of Lagny-sur-Marne.