Illarramendi was billed as the ideal replacement for Real's former midfielder Xabi Alonso, who has since joined Bayern Munich, when they paid Sociedad close to 40 million euros ($45.6 million) to secure his services in 2013.
However, he has had little playing time at the world's richest club by income, with the likes of Germany's Toni Kroos and Luka Modric of Croatia ahead of him in the pecking order.
Following the arrival last week of another Croatia international, Mateo Kovacic, who plays in a similar position, it was clear one of Real's midfielders had to go.
Spanish media reported Sociedad had agreed to pay around 17 million euros to bring Illarramendi back to San Sebastian, less than half what they made from selling him to Real.
"I am very pleased, it is what I wanted," Illarramendi was quoted as saying on Sociedad's website (www.realsociedad.com). "To meet up with my team mates again and do something good this season."
Illarramendi was in the Spain squad that won the Under-21 European championship in Israel in 2013 and was named in UEFA's team of the tournament but has yet to make an appearance for the European champions' senior side.
His return to Sociedad could revive his career and may help coach David Moyes towards his goal of taking the club back into European competition.
Sociedad began their La Liga campaign at the weekend with a 0-0 draw at Deportivo La Coruna.
(Reporting by Iain Rogers, editing by Ed Osmond)
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