Diego is a Spanish masculine given name. The Portuguese equivalent is Diogo. The etymology of Diego is disputed, with two major origin hypotheses: Tiago and Didacus. The name also has several patronymic derivations, listed below. Etymology. Tiago hypothesis. Diego has long been interpreted as variant of Tiago (also spelled as Thiago), an abbreviation of Santiago, from the older Sant Yago "Saint Jacob", in English known as Saint James or as San-Tiago (cf. San Diego).
https://en.wikipedia.org/wiki/Diego