Diego Gelmírez or Xelmírez was the second bishop and first archbishop of the Catholic Archdiocese of Santiago de Compostela in Galicia, modern Spain. He is a prominent figure in the history of Galicia and an important historiographer of the Iberia of his day. Diego involved himself in many quarrels, ecclesiastical and secular, which were recounted in the Historia Compostelana, which covered his episcopacy from 1100 to 1139 and serves as a sort of gesta of the bishop's life.

The Archdiocese of Santiago de Compostela is a Latin Church archdiocese of the Catholic Church in Spain. It is the senior of the five districts in which the church divides Galicia in North-western Spain.