Saint John the Baptist in the desert was a perennially popular subject for painting from the Renaissance onwards and derives from Luke, 1.80. Here the young saint is shown in half-length, holding a bowl in his right hand and collecting water from a spring. He is glancing downwards towards his companion, the lamb, and is holding a wooden cross with an inscribed scroll wound about it. The lamb was the traditional attribute of the saint; the animal and the full text of the inscription, 'Ecce Agnus Dei', are based on St John's words on seeing the adult Christ: 'Behold the Lamb of God' (John, 1.29).

Several variants of the composition exist, and it was engraved in reverse by Girolamo Rossi il Vecchio (active 1632-64). Although for a long time thought to be by Guercino, scholars now agree that the painting is probably a late work by Reni.

The painting appears in Pyne's illustrated Royal Residences of 1819, hanging in the King's Closet at Windsor Castle (RCIN 922104).