The announcement comes only days after the Basque Country club appointed Pellegrino Matarazzo as their new head coach.
Fluent in Spanish, Maisano will fly out to join Sociedad on Christmas night and is expected to be available from the clash against Atletico Madrid on January 5.
"It's a dream come true," Maisano said.
"This opportunity honestly came out of the blue and isn't something I could have foreseen, but it's a huge chance for me at this stage of my career."
Maisano joined Sydney FC last July to serve as Ufuk Talay's right-hand man and won the 2022 A-League Men premiership as an assistant coach at Melbourne City before that.
In 2023, he was appointed as an assistant at French side Troyes AC under Australian coach Patrick Kisnorbo, with whom he worked at City.
Sydney FC second assistant coach Matt Sim will step into Maisano's role at the Sky Blues, who sit second on the ALM ladder after nine games this season.
"Sydney FC has given me the skills, experience and belief over the past 18 months to be able to step into a role like this, and I'm incredibly excited about what lies ahead," Maisano said.
Maisano is tasked with helping Sociedad avoid relegation from La Liga, with the side precariously placed in 16th out of 20 teams on the ladder.
The bottom three sides on the table after the ongoing season will join second division next year.
Recently-retired Turkish international Ömer Toprak will serve as Matarazzo's other assistant coach.
Maisano joins Kisnorbo and Ange Postecoglou among Australians coaching in top-flight European soccer leagues during recent years.
Postecoglou was fired from his post at Nottingham Forest in September after failing to win any of his first eight games.
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