Gagny (French pronunciation: [ɡaɲi] ⓘ) is a commune in the eastern suburbs of Paris, France.

The redivision of the old departments of Seine and Seine-et-Oise then made this commune a part of Seine-Saint-Denis after an administrative transfer that went into effect 1 January 1968.

Gagny had several castles, of which the most important, demolished in 1765, belonged to Dominique de Ferrari, Maître d'hôtel ordinaire of the king in 1660.

The castle of Maison-Rouge, in the Louis XIII style, was successively the property of Hocquart, marquis of Montfermeil, then in 1845, it belonged to Louis-Philippe, then in 1864 to Michel-Victor Cruchet, a sculptor and artisan from Paris.

In 1894, some time after the death of his wife, Michel-Victor Cruchet and his two children sold the Maison Rouge estate to the religious community of the Redemptorists.

During the First World War (1914-1918), the taxis requisitioned by Paris and its suburbs assembled in front of city hall.

They were charged with taking soldiers to the Front (close to Nanteuil-le-Haudouin) for repulsing the German offensive.

By the end of the nineteenth century, in certain abandoned quarries, mushrooms were already being grown in Paris.