La Real will pay €8.5m for 60 per cent of the defender's rights, with Barcelona retaining a buy-back option worth around €20m, though the club did not disclose the figures in its own announcement.
Fort watched his new side's victory from a box at Anoeta before the move was made official, becoming Real Sociedad's second signing of the summer following the arrival of Mamadou Sarr.
The club's business this window has been shaped by defensive necessity: Real Sociedad conceded more goals than any other side in LaLiga last season, prompting the club to prioritise a new centre-back and right-back.
At right-back specifically, the coaching staff decided weeks ago to send Iñaki Rupérez down to the reserve side and pursue Fort instead. Borussia Dortmund had appeared to be leading the race for the Barcelona academy graduate, but Real Sociedad ultimately persuaded both player and club to push the deal through.
Fort is set to train for the first time under Pellegrino Matarazzo on Sunday, and could be involved when Real Sociedad host Celta Vigo on Thursday - a match in which he may make his debut. Fort came through Barcelona's youth system and made his first-team debut in the 2023-24 season, starting a Champions League match against Royal Antwerp.
He went on to make 20 appearances for the first team the following season under Hansi Flick, but fell out of the picture entirely last term, spending the campaign on loan at Elche alongside fellow Barcelona player Iñaki Peña.
Despite hoping to convince Flick during this pre-season, he was unable to force his way back into the German's plans, and Barcelona have now sold him on to the Basque club instead.
The versatile 20-year-old, capable of playing at either full-back, is seen at Real Sociedad as a long-term solution rather than a stopgap, with the club keen to add both youth and depth across a defence that struggled for consistency throughout last season. His arrival, alongside Sarr's, gives Matarazzo two new options at the back inside a matter of days.