Monte Mario is the highest (139 m) hill in Rome, Italy. It lies around the northwest side of the city.
The hill was known as Mons Vaticanus or Clivus Cinnae during the ancient Roman period. The current name, according to some theories, comes from Mario Mellini, a cardinal who owned a villa and several hamlets around the hill in the 15th century. However, the hill was known as Monte Malo ("Bad Mountain") in the Middle Ages due to the murder here of patrician Giovanni Crescenzio (998), whence, it is presumed, the modern name.
The eastern part of the hill is a nature reserve, and on the west side lies the now upmarket district of the same name. Atop one hill is the church and convent of Santa Maria Rosario. On the hilltop, now occupying the site of the 15th century Villa Mellini, is the Monte Mario Observatory, part of the Rome Observatory, and the Museo Astronomico Copernicano. This location (12°27'8.4"E ) was used as the prime meridian (rather than Greenwich) for maps of Italy until the 1960s. The side of the hill was the former site of the Villa Pigneto built by Pietro da Cortona. The ruins of the structure were razed in the 19th century.
The John Felice Rome Center, one of the four campuses of Loyola University Chicago, is located on the hill on Via Massimi.
Although it is the highest hill in the modern city of Rome, Monte Mario is not one of the proverbial Seven Hills of Rome, being outside the boundaries of the ancient city.