The Mexican international, who possesses an Italian, Uruguayan and Greek passport, has reached agreement on a one-year contract at the Campo de Futbol de Vallecas and Rayo will unveil him at midday on Wednesday.
Castillo, 29, has been back in his native Mexico for the past year, having joined Pachuca and then being loaned out to Leon following his signing from Greek outfit Aris before the start of last season.
El Diablo - one of his nicknames - moved to Europe aged just 16 when he accepted an offer from Olimpiacos before moving to Shakhtar Donetsk in July 2007 for a reported fee of €20m.
However, his spell in Russia was a disappointment and after struggling to hold down a regular starting place the diminutive winger spent the next four years on loan at Manchester City, Dnipro and Chicago Fire.
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