The figure depicted here is recognizable as Saint John the Baptist by his clothing, the lamb that accompanies him, the cross with its banner, the stream running at his feet and the shell with water alongside him, which was his baptismal instrument. He holds a book in his hands, which was not customary in images of him but not unknown in Andalusia, as can be seen, for example, in Juan de Mesa’s sculpture at the Museum in Seville. Saint John appears before a landscape in a very frontal position. He looks towards the viewer and occupies the greater part of the pictorial surface. These formal characteristics are directly related to the painting’s origin as the principal panel of an altarpiece at the vestry of the church of the Trinitarios Calzados of Cordoba. In the 19th century, the images from that altarpiece were moved to the church of Santa Marina and at least four of its six paintings were scattered and subsequently replaced with copies. The depiction of Saint John is at least twice as tall as the other works and the saint’s scale is nearly three times that of his companions, being close to life sized. The difference of scale, frontal presentation and forceful depiction of Saint John’s anatomy imbued the altarpiece with a notable monumentality that is still felt when contemplating this work alone. The manner in which the composition is resolved and the use of a subtle chiaroscuro to model the saint’s body link this painting to Zurbarán’s works, making it an example of that artist’s impact in the Guadalquivir Valley. A comparison with another Saint John the Baptist by Castillo, documented in 1643 (Cordoba, church of Santa Marina), suggests that the present work was painted at least two years later, as it is more mature in its compositional and descriptive technique. The care that Castillo took in executing this painting is revealed by the existence of various drawings. With this Saint John the Baptist, the Museo del Prado expands its collection of works by Antonio del Castillo to include a masterpiece from one of the most productive moments in his career.