Ax-les-Thermes (French pronunciation: [aks le tɛʁm]; Occitan: Ax or Acs) is a commune in the Ariège department in the Occitanie region of southwestern France.

[4] The springs were developed in the medieval period on the orders of Saint Louis to treat soldiers returning from the Crusades afflicted with leprosy.

The Bassin des Ladres (Lepers' Pond), in the centre of the commune, is fed by hot springs supplying water from the ground at a temperature of 77 °C.

Every year on St John's Day the people of Ax-les-Thermes, having been daubed with ashes, bathe in the Lepers' Pond for fun.

[8] Local economic activity is based mainly on livestock (cattle and sheep), Hydrotherapy (there are 60 sources at temperatures ranging from 18 to 78 °C, feeding three spas: Couloubret, Modèle, and Teich.

The Ax 3 Domaines winter sports resort is situated in the commune and served as the finish line for stage 14 of the 2010 Tour de France.