The 38-year-old has worked as a coach since hanging up his gloves in 2017, having taken on roles at his former club as well as Moratalaz. He left his role as academy coach at Barcelona in October 2019.
"We hope you have lots of success at the head of our club," the Spanish minnows announced on social media.
Unio Atletica d'Horta's stadium has a capacity of just 2,500 and the club have never competed at a higher level than the third tier.
Valdes left Barca in 2014. After two years at Manchester United and a brief loan spell at Belgian side Standard Liege, the Spanish World Cup winner retired after playing for Middlesbrough three years ago.