Seine-et-Marne (French pronunciation: [sɛn e maʁn] ⓘ) is a department in the Île-de-France region in Northern France.

Seine-et-Marne is one of the original 83 departments created on 4 March 1790 during the French Revolution in application of the law of 22 December 1789.

It is bordered by Val-d'Oise, Seine-Saint-Denis, Val-de-Marne, Essonne to the west; Loiret and Yonne to the south; Aube and Marne to the east; and Aisne and Oise to the north.

The department comprises part of Paris's outer eastern suburbs; much of Charles de Gaulle Airport sits within its far northwestern boundaries, including a majority of the terminals.

Over the past 50 years, however, its population has tripled, due to the development of the Paris conurbation and the building of new towns in the northwest of the region.

Seine-et-Marne as a whole shares a sister city relationship with Orlando, Florida, United States, as both host Disney theme parks.

The other key industrial structures are the refinery at Grandpuits and the Safran Aircraft Engines research plant at Villaroche.