The Palazzo San Callisto (also known as the Palace of Saint Callixtus) is a small palace in Rome and one of the extraterritorial Properties of the Holy See. The original Palazzo is located in the Piazza di Santa Maria in Trastevere, the later extensions have their entrance in Piazza di San Callisto. The entire complex is one of the areas of the Holy See regulated by the 1929 Lateran Treaty signed with the Kingdom of Italy.