The Marne (French pronunciation: [maʁn] (listen)) is a river in France, an eastern tributary of the Seine in the area east and southeast of Paris. It is 514 kilometres (319 mi) long. The river gave its name to the departments of Haute-Marne, Marne, Seine-et-Marne, and Val-de-Marne.

The Marne starts in the Langres plateau, runs generally north then bends west between Saint-Dizier and Châlons-en-Champagne, joining the Seine at Charenton just upstream from Paris. Its main tributaries are the Rognon, the Blaise, the Saulx, the Ourcq, the Petit Morin and the Grand Morin.

Near the town of Saint-Dizier, part of the flow is diverted through the artificial Lake Der-Chantecoq. This ensures both flood prevention and the maintenance of minimum river flows in periods of drought.

The Marne is famous as the site of two eponymous battles during World War I. The first battle was a turning point of the war, fought in 1914. The second battle was fought four years later, in 1918.