Guardiola, no relation to Pep, started the season with La Liga side Granada, but is yet to get any playing time with the first team.
The 25-year-old joins fellow Spaniards Isaias, Sergio Cirio and coach Guillermo Amor at the A-League champions.
Guardiola is probably best known for having one of the shortest contracts in FC Barcelona history, after he was released just a few hours after signing with the Catalan club in December 2015.
The striker signed with Barca's B team on a one-and-half-year deal only to be released a few hours later after it was discovered he had tweeted offensive anti-Catalan statements in 2013.
Guardiola's best record at a club came when he scored 14 goals in 41 matches in the Spanish third division for Eldense.
With Guardiola's signing Adelaide United have filled their import quota of five who joins Isaias, Cirio, Italian Iacopo La Rocca and Argentine Marcelo Carrusca.
