El Escorial, or the Royal Site of San Lorenzo de El Escorial (Spanish: Monasterio y Sitio de El Escorial en Madrid), or Monasterio de El Escorial (Spanish pronunciation: [el eskoˈɾjal]), is a historical residence of the king of Spain located in the town of San Lorenzo de El Escorial, 2.06 kilometres (1.28 mi) up the valley (4.1 km [2.5 mi] road distance) from the town of El Escorial and about 45 kilometres (28 mi) northwest of the Spanish capital Madrid. Built between 1563 and 1584 by order of King Philip II (who reigned 1556–1598), El Escorial is the largest Renaissance building in the world. It is one of the Spanish royal sites and functions as a monastery, basilica, royal palace, pantheon, library, museum, university, school, and hospital.

El Escorial consists of two architectural complexes of great historical and cultural significance: the royal monastery itself and La Granjilla de La Fresneda, a royal hunting lodge and monastic retreat about five kilometres (3.1 mi) away. These sites have a dual nature: during the 16th and 17th centuries, they were places in which the power of the Spanish monarchy and the ecclesiastical predominance of the Roman Catholic religion in Spain found a common architectural manifestation. El Escorial was both a Spanish royal palace and a monastery. Established with a community of Hieronymite monks, it has become a monastery of the Order of Saint Augustine. It also contained a boarding school, now the Real Colegio de Alfonso XII, still in operation.

Philip II engaged the Spanish architect Juan Bautista de Toledo to be his collaborator in the building of the complex at El Escorial. Toledo had spent the greater part of his career in Rome, where he had worked on St. Peter's Basilica, and in Naples serving the king's viceroy, whose recommendation brought him to the king's attention. Philip appointed him architect-royal in 1559, and, together, they designed El Escorial as a monument to Spain's role as a center of the Christian world.

On 2 November 1984, UNESCO declared The Royal Seat of San Lorenzo of El Escorial a World Heritage Site. It is a popular tourist attraction, often visited by day-trippers from Madrid—more than 500,000 visitors come to El Escorial every year.

El Escorial is situated at the foot of Mount Abantos in the Sierra de Guadarrama. This austere location, hardly an obvious choice for the site of a royal palace, was chosen by King Philip II of Spain, and it was he who ordained the building of a grand edifice here to commemorate the 1557 Spanish victory at the Battle of St. Quentin in Picardy against King Henry II of France. King Phillip II's goal was to have a princely mansion in a wooded countryside, while also being architecturally different from most buildings that were being commissioned at the time. Phillip's decision to do this was inspired by his journeys in Europe exploring architecture, including the Ospedale Maggiore that inspired ideas for the design of the Escorial. One of Phillip II's personal aspirations was to be a monk and a monarch, and this would come to influence the design. The result of all these ideas was an imposing three story tall, orthogonal, stronghold-like structure with a dual function as both a royal residence and monastery. Philip also intended the complex to serve as a necropolis for the interment of the remains of his parents, Charles I and Isabella of Portugal, himself, and his descendants. In addition, Philip envisioned El Escorial as a center for studies in aid of the Counter-Reformation cause. The Escorial would come to be so iconic that the design of the building became a new architectural style of Spanish renaissance architecture, known as the Herrerian style.

The building's cornerstone was laid on 23 April 1563. The design and construction were overseen by Juan Bautista de Toledo, who did not live to see the completion of the project. With Toledo's death in 1567, direction passed to his apprentice, Juan de Herrera, under whom the building was completed in 1584, in slightly less than 21 years. To this day, la obra de El Escorial ("the work of El Escorial") is a proverbial expression for a thing that takes a long time to finish (comparable English proverb: "Rome wasn't built in a day").

Since then, El Escorial has been the burial site for most of the Spanish kings of the last five centuries, Bourbons as well as Habsburgs. The Royal Pantheon contains the tombs of the Holy Roman Emperor Charles V (who ruled Spain as King Charles I), Philip II, Philip III, Philip IV, Charles II, Louis I, Charles III, Charles IV, Ferdinand VII, Isabella II, Alfonso XII, and Alfonso XIII. Two Bourbon kings, Philip V (who reigned from 1700 to 1724 and again from 1724 to 1746) and Ferdinand VI (1746–1759), as well as King Amadeus (1870–1873), are not buried in the monastery.

The floor plan of the building is in the form of a gridiron. The traditional belief is that this design was chosen in honor of Saint Lawrence, who, in the third century AD, was martyred by being roasted to death on a grill. St. Lawrence's feast day is 10 August, the same date as the 1557 Battle of St. Quentin.