The 20-year-old Spaniard has agreed a five-year deal at Camp Nou that features a release clause worth €400m.
Garcia will sign his contract on Tuesday afternoon before he is presented as Barca's latest acquisition at a press conference.
The Barcelona-born defender is returning to the club whose academy he elected to leave at the age of 16 to join Pep Guardiola at City.
In the four years since, Garcia progressed through the ranks at the Etihad Stadium and forced his way into the first team.
However, his refusal to sign a new contract saw him fall behind Ruben Dias and John Stones in the pecking order in the 2020-21 season.
Home is where the heart is. pic.twitter.com/FQRV2L4n4R— FC Barcelona (@FCBarcelona) June 1, 2021
And the reasons for that decision have now become clear, with the Spain international heading back to his native country.
Garcia's stay in England ends with him having made 35 appearances in all competitions, including 19 Premier League outings.
The centre-back becomes the second player to leave City for Barca at the end of his contract this summer after Sergio Aguero's move between the clubs was confirmed this week.
Ronald Koeman is expected to secure another Premier League free transfer in departing Liverpool midfielder Georginio Wijnaldum.
Memphis Depay, whose deal with Lyon is also on the brink of expiry, is also thought to be close to joining the cash-strapped Catalans.