Rodrigo Díaz de Vivar was a Castilian knight and ruler in medieval Spain. Fighting both with Christian and Muslim armies during his lifetime, he earned the Arabic honorific as-Sayyid ("the Lord" or "the Master"), which would evolve into El Çid (Spanish: [el ˈθið], Old Spanish: [el ˈts̻id]), and the Spanish honorific El Campeador ("the Champion"). He was born in Vivar, a village near the city of Burgos.

As the head of his loyal knights, he came to dominate the Levante of the Iberian Peninsula at the end of the 11th century. He reclaimed the Taifa of Valencia from Moorish control for a brief period during the Reconquista, ruling the Principality of Valencia from 17 June 1094 until his death in 1099. His wife, Jimena Díaz, inherited the city and maintained it until 1102 when it was reconquered by the Moors.

Díaz de Vivar became well known for his service in the armies of both Christian and Muslim rulers. After his death, El Cid became Spain's most celebrated national hero and the protagonist of the most significant medieval Spanish epic poem, El Cantar de mio Cid, which presents him as the ideal medieval knight: strong, valiant, loyal, just, and pious.

There are various theories on his family history, which remains uncertain; however, he was the grandfather of García Ramírez de Pamplona, King of Navarre, the first son of his daughter Cristina Rodríguez. To this day, El Cid remains a popular Spanish folk hero and national icon, with his life and deeds remembered in popular culture.