Castres is the sole subprefecture of the Tarn department in the Occitanie region in Southern France. It lies in the former province of Languedoc, although not in the former region of Languedoc-Roussillon. In 2018, the commune had a population of 41,795. Castres is the fourth-largest industrial centre of the predominantly rural former Midi-Pyrénées region after Toulouse, Tarbes and Albi, as well as the largest in the part of Languedoc lying between Toulouse and Montpellier.