Segundo Alejandro Castillo Nazareno (born 15 May 1982) is an Ecuadorian international footballer who plays for Guayaquil City of the Ecuadorian Serie A. Castillo is mostly used as a defensive midfielder but can also play as a central midfielder.
Born in San Lorenzo, Esmeraldas, Castillo started his career with Quito's Club Deportivo Espoli spending 3 years with the team, scoring a decent 11 goals in 66 appearances. He then moved across the capital to one of Ecuador's most successful sides Club Deportivo El Nacional, with whom he won successive league titles in 2005 and 2006. He again scored 11 goals in 112 matches for the club.
In August 2006 he moved abroad, signing a two-year deal with Serbian side Red Star Belgrade. He scored 8 goals during his first season at his new club and was a key player when the team won the double in 2007. In total he scored 18 goals in 72 games, an average of a goal every four games.