Monreale Cathedral (Italian: Cattedrale di Santa Maria Nuova di Monreale; Duomo di Monreale) is a Catholic church in Monreale, a municipality in the Metropolitan City of Palermo, Sicily. One of the greatest extant examples of Norman architecture, it was begun in 1174 by William II of Sicily and is dedicated to the Nativity of the Virgin Mary. The church was elevated by a bull of Pope Lucius III to the rank of a metropolitan cathedral in 1182 as the seat of the diocese of Monreale, which was elevated to the Archdiocese of Monreale in 1183. Since 2015 it has been part of the Arab-Norman Palermo and the Cathedral Churches of Cefalù and Monreale UNESCO World Heritage Site.

The church is a national monument of Italy and one of the most important attractions of Sicily. It is 102 meters (335 feet) long and 40 meters (131 feet) wide.

According to a legend, William II of Sicily fell asleep under a carob tree while hunting in the woods near Monreale. The Holy Virgin appeared to him in his dreams and suggested that he build a church where he lay. While clearing the land, a treasure trove of gold coins was found in the roots of the carob tree, which were later used to finance the construction of the structure. It is more likely that the church was part of a plan of large constructions in competition with the then bishop of Palermo, Walter Ophamil, who had ordered the large Cathedral of Palermo. The construction of Monreale, started in 1172, was approved by Pope Alexander III with a bull on 30 December 1174. Works, including an annexed abbey, were completed only in 1267 and the church consecrated at the presence of Pope Clement IV. In 1178 Pope Lucius III established the archdiocese of Monreale and the abbey church was elevated to the rank of cathedral. The archbishops obtained by the kings of Sicily a wide array of privileges and lands in the whole Italian peninsula.

In 1270 the heart of Saint Louis IX, King of France, brother of King Charles I of Naples, was buried here.

In 1547-1569 a portico was added to the northern side, designed by Giovanni Domenico Gagini and Fazio Gagini, in Renaissance style, covered by a cross vault and featuring eleven round arches supported by Corinthian columns. In 1559 most of the internal pavement was added.

The archiepiscopal palace and monastic buildings on the south side were of great size and magnificence, and were surrounded by a massive precinct wall, crowned at intervals by twelve towers. This has been mostly rebuilt, and but little now remains except ruins of some of the towers, a great part of the monks' dormitory and frater, and the splendid cloister, completed about 1200.

The latter is well preserved, and is one of the finest European cloisters extant both for its size and the beauty of its detailing. It is about 2,200 m², with pointed arches decorated with diaper work, supported on pairs of columns in white marble, 216 in all, which are alternately plain and decorated with bands of patterns in gold and colors, made of glass tesserae, arranged either spirally or vertically along each shaft. The marble capitals are each carved with foliage, biblical scenes and allegories; no two are the same. At one corner, a square pillared projection contains the marble fountain or monks' lavatorium.

The plan of the church is based on models of Clunian origin from northern France. The facade is flanked by two towers, like Cefalù Cathedral, while the large three-apse choir is similar to one of the first three-apse churches.