Sandro has made 134 appearances for the Bianconeri since his arrival at the club from FC Porto in 2015.
As revealed by Goal earlier this month, his new deal is expected to see him earn a base wage of around €4 million (£4m/$5m) per season.
Sandro, whose previous deal ran until the summer of 2020, has been part of three Scudetto winning sides along with three Coppa Italia crowns with the Turin club.
Former Manchester United boss Jose Mourinho is said to have been a big fan of Sandro and, in September, Sandro said that Juve had not contacted him about a new contract, claiming "a proposal has not yet come from [Juventus] to renew. I think only of the present and the field".
But a new four-and-a-half-year deal has been in the pipeline for several weeks and the Brazilian finally put pen to paper on Thursday.
Sandro has made 20 appearances for Juventus so far this season and is a mainstay in Massimiliano Allegri's side, who sit top of the Serie A table and remain unbeaten in the league.
Sandro featured in Juve's 4-1 Champions League defeat to Real Madrid in the 2016-17 final in Cardiff.
Next up for Juventus is a league tie against Eusebio Di Francesco's Roma, who are currently seventh in the table and 22 points behind the Bianconeri.