Castelsarrasin is a commune located in the Tarn-et-Garonne department within the Occitanie region of southwestern France. The residents of the commune are known as Castelsarrasinois.
It ranks as the second most populous commune in the Tarn-et-Garonne department, following Montauban. The town is served by the Castelsarrasin station, which provides access to the Bordeaux-Toulouse railway line.