Sancho III of Castile (1134 – August 30, 1158), called el Deseado (The Desired), was King of Castile for one year, from 1157 to 1158.

He was the eldest son of King Alfonso VII of Castile and Berenguela of Barcelona. His father's will partitioned the kingdom between his two sons: Sancho inherited Castile, and Fernando inherited Leon. The two brothers had just signed a treaty before Sancho's sudden death in the summer of 1158. He left an only son and heir, Alfonso VIII of Castile, by his wife Blanca of Navarre.