Inês de Castro (died January 7 1355) was a Castilian noblewoman, daughter of Pedro Fernandez de Castro.

Inês came to Portugal in 1340 as a maid of princess Constance of Castile, recently married to prince Peter, the heir to the throne. The prince fell in love with her and started to neglect his lawful wife, endangering the already feeble relations with Castile. Moreover, Peter love for Inês brought the exiled Castilian nobility very close to power: Inês's brothers became the prince’s friends and trusted advisors. King Afonso IV of Portugal, Peter’s father, disliked Inês's influence on his son and waited patiently for the "affair" to wear out. Unfortunately for the relations between the two, it didn’t. Inês and Peter kept seeing each other despite royal disapproval.

In 1349, Constance of Castile died. Afonso IV tried several times to remarry his son, but Peter refused to take a wife other than Inês—an unacceptable queen. Meanwhile, Peter’s legitimate son (future king Fernando of Portugal) was a frail child, whereas Inês's bastards where thriving. This created even more discomfort among the Portuguese nobles, scared with increasing Castilian power with Peter. Afonso IV banished her from the court after Constance's death, but Peter remained with her. After several attempts of keeping the lovers away, Afonso IV ordered Inês's death. Pero Coelho, Alvaro Gonçalves and Diogo Pacheco went to the Monastery of Santa Clara in Coimbra, where Inês was detained, and killed her. Inês's death did not bring Peter closer to his father; following his lover’s death, he rebelled against Afonso IV and dragged the country to a civil war.

Peter became king of Portugal in 1357. He then revealed to the country that had secretly married Inês and that she was the lawful queen of Portugal. The king's word was, and still is, the only proof of the marriage, but Peter took Inês's body from the grave and forced the entire court to swear allegiance to her as queen.

Inês de Castro’s life is immortalized in several plays and poems in Portuguese and Spanish language, including The Lusíadas by Camões.