The former Olympiakos and Manchester City striker will play in his native country for the first time after a 12-year career in Europe.
After 12 years abroad, the 28-year-old Castillo celebrated his most recent birthday by signing a three-year contract with the Hidalgo side.
Castillo will reunite with Hugo Sanchez, who coached him from 2007 to 2008 with El Tri. The former Olympiakos striker burst onto the international scene under Sanchez's tutelage, with a fantastic Copa América that saw Castillo score on Brazil, Ecuador and Paraguay.
His performance at the South American competition spurred an expensive transfer to Shakhtar Donetsk, where Castillo struggled. Loan spells at Manchester City, Dnipro Dnipropetrovsk and Chicago Fire did not help.
A return to Greece stabilized his form and prompted talk amongst media members that he might be recalled to the national side after a long absence.
Previously, Chivas and Cruz Azul had been named as suitors for Castillo, but it is believed that the player had turned them down in an effort to remain in European soccer.
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