The Royal Collegiate Basilica of San Isidoro, commonly known as San Isidoro de León, is a Christian church located in the city of León. It is considered one of the most outstanding Romanesque complexes in the country, both for its historical significance and for its architecture, sculpture, and the valuable collection of Romanesque ceremonial objects that have survived to the present day. One of its most distinctive features is the Royal Pantheon, located at the foot of the church, which preserves original Romanesque murals and capitals, making it an exceptional example of European Romanesque art. The complex was built and expanded primarily between the 11th and 12th centuries.

Originally, the site housed a monastery dedicated to Saint Pelayo, although it is believed that a Roman-era temple previously stood there. With the arrival in León of the relics of Saint Isidore, Bishop of Seville and Doctor of Spain, the building adopted its current dedication.

The church preserves Romanesque remains from the first construction phase initiated by King Ferdinand I and Queen Sancha. The Royal Pantheon and the doors of the south façade—the Puerta del Cordero and the Puerta del Perdón—along with the North Door or Capitular Door, represent the earliest expressions of Romanesque art in the León region. Over the centuries, the complex incorporated elements of the Gothic, Renaissance, and Baroque styles.

Since February 9, 1910, San Isidoro de León has been recognized as a Historic-Artistic Monument.