St. Isidore's Basilica (Spanish: Basílica de San Isidoro) is a Catholic church in the city of León, Castile and León, Spain, located on the site of an ancient Roman temple. Its Christian roots can be traced back to the early 10th century when a monastery for Saint John the Baptist was erected on the grounds.

In 1063 the basilica was rededicated to Saint Isidore of Seville. Isidore was archbishop of Seville, and the most celebrated academic and theologian of Visigothic Spain in the period preceding the Arab invasions. With the agreement of Abbad II al-Mu'tadid, the Muslim ruler of Seville, Isidore's relics were brought to Leon where they could be interred on Christian soil. The tomb of the saint still draws many visitors today. An equestrian statue of Saint Isidore dressed as a Moor-slayer is visible, along with many other sculptures, high on the facade.

In 1188, the Cortes of León were held in here. It was the first sample of modern parliamentarism in the history of Europe, according to the UNESCO and John Keane's book The Life and Death of Democracy.