For the video game series, see Gex.

Gex is a commune and the chief town of an arrondissement in the Ain département, France, 5 km from the Swiss border, and 16 km from Geneva.

The town gave its name to the Pays de Gex, at various times under the jurisdiction of France, Switzerland and the Counts of Savoy. Exports significantly to Geneva.

With the 1815 Treaty of Vienna Gex was placed in the Customs region of Switzerland and neutralised. The resolutions regarding the Pays de Gex were annulled by Art 435 of the Treaty of Versailles. In November 1923 France moved its customs office to Gex, and the matter was brought before the Permanent Court of International Justice (predecessor of the International Court of Justice), which decided in favour of Switzerland. A compromise was reached in 1932.