The 24-year-old was already under contract at the Anoeta until 2004, but Real have decided to offer their youth product a new three-year deal, with an option for further season.
Gabilondo was by no means a regular starter under Raynald Denoueix last term as Sociedad finished second in La Liga, making eight starts in the league, and a further 16 as substitute.
However, with Champions League football assured, Gabilondo is expected to play a more prominent role for the club next term.
Meanwhile, Real are believed to have drawn up a list of players they hope to bring to the club during the summer.
Their priorities are midfielder Bittor Alkiza, who recently left Basque rivals Athletic Bilbao, Osasuna defender Cesar Cruchaga, and also Croatian striker Ivica Olic.
