Koeman lasted just 14 months in the job after replacing Quique Setien in August 2020 and departs Camp Nou with a 58.21 win percentage across his 67 games in charge.
Widespread reports from Spain suggest legendary Barcelona midfielder Xavi, who is in charge of Qatari side Al Sadd, is being lined up to replace the former Netherlands boss as early as next week.
Talks between the two parties are said to be ongoing, but Barca have moved to place Sergi in charge for the time being.
The former left-back, who has spent the past four months at the helm of Barcelona's B side, will be introduced to the first-team squad ahead of Friday's training session.
He has also previously managed Zhejiang Greentown, Real Mallorca, Almeria and Recreativo following a 15-year playing career that saw him spend nine seasons at Camp Nou.
The LaLiga side confirmed the news on its official website, with Sergi set to speak to the media for the first time on Friday ahead of this weekend's visit of Deportivo Alaves.
Barca then has a crucial Champions League trip to Dynamo Kiev before travelling to Celta Vigo in its final league game before the international break.