Coordinates: 49°02′10″N 2°03′47″E / 49.0361°N 2.0631°E / 49.0361; 2.0631

Cergy (French pronunciation: [sɛʁ.ʒi]) is a commune in the northwestern suburbs of Paris, France. It is located 27.8 km (17.3 mi) from the center of Paris, in the "new town" of Cergy-Pontoise, created in the 1960s, of which it is the central and most populated commune.

Although neighboring Pontoise is the official préfecture (capital) of the Val-d'Oise département, the préfecture building and administration, as well as the department council (conseil général), are located inside the commune of Cergy, which is regarded as the de facto capital of Val-d'Oise. The sous-préfecture building and administration, on the other hand, are located inside the commune of Pontoise.

The name Cergy comes from Medieval Latin Sergiacum, meaning "estate of Sergius", a Gallo-Roman landowner.

Cergy is the chief town of two cantons:

When Cergy was selected to become the center of a "new town", it was only a village. The commune had only 2,895 inhabitants in 1968. It then started to develop very quickly, exceeding 10,000 inhabitants in the mid-1970s and then 20,000 in the early-1980s. It is in this decade that its growth was most spectacular, since the city exceeded 48,000 inhabitants in 1990. The increase continued since, but at a notably slower pace, to reach 54,500 as of 2004 estimates. However, on 1 January 2000, the commune lost a portion of its territory (net 0.03 km² (0.012 sq mi) with a 1999 population of 62 persons) to the adjacent commune of Courdimanche. The official census figures have thus been revised downward from the 1999 official 54,781 to 54,719, and the land area from 11.68 km² (4.51 sq mi) to 11.65 km².