Marne (French pronunciation: [maʁn]) is a department in the Grand Est region of France. It is named after the river Marne which flows through it. The prefecture (capital) of Marne is Châlons-en-Champagne (formerly known as Châlons-sur-Marne). The subprefectures are Épernay, Reims, and Vitry-le-François. It had a population of 566,855 in 2019.

The Champagne vineyards producing the eponymous sparkling wine are in Marne.

The department is named after the Marne, which was called Matrona in Roman times.

Marne is one of the original 83 departments created during the French Revolution on March 4, 1790. It was created from the province of Champagne.

Marne has a long association with the French Army. The training ground of the Camp Militaire de Mailly straddles the border with the département of Aube in the south while that of the Camp de Mourmelon occupies a large area north of Châlons-en-Champagne. The smaller Camp de Moronvilliers lies to the east of Reims and the Camp Militaire de Suippes lies to the east of that. These are all located on the chalk grounds of the Champagne plateau, a feature comparable in geology but not size, with the British military training ground on Salisbury Plain.

The Battles of the Marne, where the British and French fought against Germany during World War I, took place here.

Marne is part of the region of Grand Est and is surrounded by the departments of Ardennes, Meuse, Haute-Marne, Aube, Seine-et-Marne, and Aisne.

Geologically, it divides into two distinct parts; the Upper Cretaceous chalk plain in the east and the more wooded and hilly Eocene and Oligocene in the west.