Soisy-sous-Montmorency (French pronunciation: [swazi su mɔ̃mɔʁɑ̃si] ⓘ, literally Soisy under Montmorency) is a commune in the Val-d'Oise département located 15.3 km (9.5 mi) north of Paris, France.

On 7 August 1850, a part of the territory of Soisy-sous-Montmorency was detached and merged with a part of the territory of Saint-Gratien, a part of the territory of Deuil-la-Barre, and a part of the territory of Épinay-sur-Seine to create the commune of Enghien-les-Bains.

Soisy-sous-Montmorency is served by Champ de courses d'Enghien station on the SNCF Transilien Paris – Nord suburban rail line.

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