The 30-year-old signed a four-year contract with the defending La Liga champion after a successful stint in Italian Serie A.
He has helped Juventus win three league titles in his four years at the club and looks set to add another this season with a four-point lead over the top team.
Veteran duo contracts
The decision of the Bosnian international will see Arthur of Barcelona head in the other direction, Juventus paying a first 80 million dollars to the Brazilian.
Arthur joined Barcelona from Brazilian club Grêmio in 2018, winning the championship title and the Super Cup in his first season.
Like Pjanić, the 23-year-old will not move to his new club until the end of the current season.
Juventus also agreed to a one-year contract extension for 35-year-old captain Giorgio Chiellini and 42-year-old goalkeeper Gianluigi Buffon.
The veteran goalkeeper has played more than 500 times for Juventus since his debut in 2001 and returned to the Italian squad last year after a stint with the French champion PSG.