Saint John the Evangelist, the youngest disciple of Jesus, is depicted in profile with a chalice in hand from which a serpent emerges. The image recalls a legendary episode of the saint's life: he was offered a chalice of poisoned wine to drink but Saint John blessed the cup so that the poison turned into a serpent that then emerged from it.
The suffuse light from the right illuminates a bare, dark setting, creating an intimate, introspective atmosphere that emphasises the graceful features of the young apostle absorbed in meditation and creates evocative reflections on the golden chalice.  
The painting became known in 1998 when it appeared on the antiques market and was recognised by scholars as a work signed by Guercino, belonging the late period of his production, at the start of the sixteen fifties. It may be a “Saint John” painted for Giuseppe Maria Galeppini, an artist from Forlì who came into contact with Guercino through Bartolomeo Gennari, whose pupil he was.