Cergy (French pronunciation: [sɛʁʒi] ) is a commune in the French department of Val-d'Oise, to the northwest of Paris. It is located 27.8 km (17.3 mi) from the centre of Paris, in the "new town" of Cergy-Pontoise, created in the 1960s, of which it is the central and most populated commune.

Although neighbouring 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: Cergy-1 and Cergy-2.

• Columbia, Maryland, United States.
• Erkrath, Germany.
• Liaoyang, People's Republic of China.
• Porto Novo, Benin.
• Tres Cantos, Spain
• West Lancashire, United Kingdom.

Cergy is also informally twinned with a village in Palestine and a village in Senegal.

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 at 2004 estimates. However, in December 2001, the commune lost a portion of its territory (net 0.03 km (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 km (4.51 sq mi) to 11.65 km.

Historical population

Year Pop. ±% p.a. 1793 1,045 —     1800 1,084 +0.52% 1806 1,069 −0.23% 1821 987 −0.53% 1831 1,016 +0.29% 1836 1,000 −0.32% 1841 970 −0.61% 1846 956 −0.29% 1851 963 +0.15% 1856 922 −0.87% 1861 933 +0.24% 1866 953 +0.43% 1872 926 −0.48% 1876 897 −0.79% 1881 869 −0.63% 1886 889 +0.46% 1891 897 +0.18% 1896 902 +0.11%	Year Pop. ±% p.a. 1901 885 −0.38% 1906 942 +1.26% 1911 933 −0.19% 1921 963 +0.32% 1926 1,060 +1.94% 1931 1,118 +1.07% 1936 1,106 −0.22% 1946 1,138 +0.29% 1954 1,447 +3.05% 1962 2,455 +6.83% 1968 2,895 +2.79% 1975 8,896 +17.39% 1982 19,357 +11.75% 1990 48,226 +12.09% 1999 54,781 +1.43% 2007 57,600 +0.63% 2012 60,528 +1.00% 2017 65,177 +1.49%
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Source: EHESS and INSEE (1968-2017)

Place of birth of residents of Cergy in 1999

Born in metropolitan France	Born outside metropolitan France
78.2%	21.8%
	Born in overseas France	Born in foreign countries with French citizenship at birth	EU-15 immigrants	Non-EU-15 immigrants
	2.8%	3.4%	1.9%	13.7%
This group is made up largely of former French settlers, such as pieds-noirs in Northwest Africa, followed by former colonial citizens who had French citizenship at birth (such as was often the case for the native elite in French colonies), as well as to a lesser extent foreign-born children of French expatriates. A foreign country is understood as a country not part of France in 1999, so a person born for example in 1950 in Algeria, when Algeria was an integral part of France, is nonetheless listed as a person born in a foreign country in French statistics. An immigrant is a person born in a foreign country not having French citizenship at birth. An immigrant may have acquired French citizenship since moving to France, but is still considered an immigrant in French statistics. On the other hand, persons born in France with foreign citizenship (the children of immigrants) are not listed as immigrants.

Port Cergy is a marina on the River Oise at pk 9. The site comprises both housing and recreational yachts as well as a boating school. The marina can hold 103 ships ranging 5 to 22m long and. The northern part of the site is reserved for restaurants and shops and have been built around a small bain.

Ham is a small village to the south of Cergy. The village used to be part of the Sergentery and then commune of Neuville-sur-Oise but has since been amalgamated in the commune of Cergy. On its territory is the outdoor leisure centre 'Base de Loisirs de Cergy-Neuville'.

Cergy is served by three stations on Paris RER line A and on the Transilien Paris – Saint-Lazare suburban rail line: Cergy-Préfecture, Cergy-Saint-Christophe, and Cergy-le-Haut. Cergy is served by direct buses from Charles de Gaulle Airport.

The bus company STIVO provides 17 lines of buses to travel within the agglomeration of Cergy.

Secondary schools:

• Junior high schools (collèges): Gérard Philipe, La Justice, Les Explorateurs, Les Touleuses, Moulin à Vent
• Senior high schools/sixth-form colleges: Lycée polyvalent Galilée and Lycée polyvalent Jules Verne

Lycée Alfred Kastler de Cergy-Pontoise is in neighbouring Pontoise.

Tertiary education:

• Cergy-Pontoise University
• CY Tech, formerly EISTI (École internationale des sciences du traitement de l'information)
• ESSEC Business School (grande école)
• ENSEA École Nationale Supérieure de l'Électronique et de ses Applications
• ITIN, IT-Institute (École supérieure d'informatique, réseaux et systèmes d'information)
• Web site of Professional Bachelor’s degree in International Trade and International Tourism (Cergy-Pontoise University)

Known as being a violent city in the past, with a criminal rate of 137.62 incidents per 1000 inhabitants, Cergy-Pontoise has experienced a significant decrease of violence in the past years. Reaching, in the year of 2008, a rate of 9,87 criminal incidents per 1000 inhabitants, which is still a high criminal rate.

The town was used as a filming location for Henri Verneuil's film I... comme Icare released in 1979, starring Yves Montand. The EDF-GDF tower designed by architect Renzo Moro is the building from which the shots were fired to assassinate president Marc Jarry. The country that the movie depicts is not named, although the United States is perhaps suggested. The filmmakers chose the modern and innovative architecture of the new city to avoid depicting any particular country.

The town was also the location of the 1987 film My Girlfriend's Boyfriend directed by Eric Rohmer.

• Communes of the Val-d'Oise department

• Official website (in French)
• Base Mérimée: Search for heritage in the commune, Ministère français de la Culture. (in French)
• Port Cergy (in English)