The loan, without a purchase option, arrives after several other targets for the position β including Ladislav KrejΔΓ, Nayef Aguerd and Jonathan Jesus β fell through over the course of the summer, according to Diario Vasco.
Sarr will not be available for Real Sociedad's fixture on Monday night as the paperwork is finalised, but the move is expected to be completed in the coming days.
The 20-year-old came through Olympique Lyonnais's academy before a loan spell at Belgian side RWD Molenbeek, then joined Strasbourg in Ligue 1 for the 2024-25 season, where his form as a teenager caught wider attention.
Chelsea moved for him roughly a year later in a deal reported to be worth around β¬14million, though he continued out on loan at Strasbourg for the first half of last season before being recalled by the London club in the winter window. He has featured sparingly since, making only seven appearances for Chelsea, and does not figure in Xabi Alonso's plans this season.
Journalist Santi Aouna, the same reporter who first revealed the collapse of talks over Aguerd, broke the news of Real Sociedad's interest in Sarr a few days ago, and Fabrizio Romano has since confirmed the clubs have reached a verbal agreement.
The move follows an unsuccessful trip to Brazil by sporting director Jokin Aperribay and Erik Bretos, who had travelled in an attempt to sign Jonathan Jesus before turning their attention back to Sarr.
The arrival of a centre-back who turns 21 on 29 August carries an element of risk for Real Sociedad given the club's current stance on Jon Pacheco, who at almost 26 is not yet considered a guaranteed starter, while Luken Beitia and Kazunari Kita β both of whom impressed for the reserve side, Sanse, last season β are not thought ready to be relied upon at first-team level either.
Sarr's physical profile has been central to his appeal, standing 1.94 metres tall and combining that frame with a comfort in possession that has drawn comparisons to more modern, ball-playing defenders rather than a purely combative centre-back.
La Real great Xabi Alonso is understood to have played a role in sanctioning the loan, having worked with Sarr since taking charge at Stamford Bridge and viewing the move as a chance for the young Frenchman to gain regular first-team experience in La Liga.