Diego is a Spanish male name derived from the Hebrew Yaʻăqōbh (Jacob), via the name of Saint James the Greater (Sant Yago), re-analysed as Santiago and San Diego. The assimilation of the final t of Sant into the name is attested in other saints' names, including Spanish and Portuguese Telmo 'Elmo' (< Sant Elmo) and Slovenian Tilen 'Giles' (< Šent Ilen).
Diego is a very popular name in the Spanish-speaking world. In Brazil/Portugal, France and Italy it is a very widespread name as well (as are other Spanish names such as Fernando). For example: due to the Spanish influence from the days of the kingdom of Naples, on the death of king Alfonso I of Naples (also known as Alfonso V of Aragon) in 1458, a famous elegy was composed by one Diego del Castillo.
The patronymic for Diego is Díaz in Castillian (used for example by Rodrigo Díaz de Vivar, better known as El Cid) and Dias in Portuguese. Like many patronymics, these have become common surnames in Spanish- and Portuguese-speaking regions. The form Diéguez is much less common; Diegues can be found in Portuguese-speaking countries. de Diego and Diego can also be found as surnames.