Juan del Castillo was a Spanish Jesuit priest and missionary who served the Guarani people in South America during the early 17th century. Born in Belmonte, Spain, he initially studied law at the University of Alcalá before joining the Society of Jesus in Madrid. After traveling to South America in 1616, he focused his ministry on the Jesuit reductions in the region of Paraguay. He is recognized as a martyr-saint within the Catholic Church for his dedication to his mission.