The patron of Chianciano Terme is Saint John the Baptist, celebrated on 24 June. His church is the collegiate church of San Giovanni Battista, which also has a museum of its own.
The left-hand chapel, known as the Baptistery chapel, holds the round travertine font of the old baptistery and the fresco of the Madonna and Child known as the Madonna del Castagnolo, by Bartolo di Fredi: both 14th-century works. On the choir loft above the entrance stands the organ built in 1771 by Ignazio Muschietti.
