— Flamengo (@Flamengo) August 19, 2020
Isla, 32, was a free agent after leaving Fenerbahce and received offers from a number of clubs.
Boca Juniors also tried to sign Isla, but in the end he chose Flamengo after they made a late bid for his services.
He will wear the number 44 shirt at the Gávea Stadium.
A centurion for Chile with 115 caps, Isla bolster a Flamengo side that has started the season slowly under new boss Domenec Torrent.
In his 13 years playing in Europe, Isla wore the shirts of Udinese, Juventus, QPR, Marseille, Cagliari and Fenerbahce.
Flamengo will be Isla’s first professional experience in South America as he left Universidad Catolica as a teenager.
Guga to join Isla?
Isla may soon be joined by another right-back with Flamengo hoping to sign Guga from Atletico Mineiro.
Atletico are asking for €5m to sell 21-year-old Guga, a Brazil under-23 international.
Talks between the two clubs are expected to accelerate now that the Isla deal has been finalised.