Barcelona midfielder Arthur Melo will join Juventus at the end of the season in a deal worth 72million euros (£66.06m).
Arthur joined the reigning LaLiga champions from Gremio in 2018 and has gone on to make 72 appearances for the club, winning the league title and the Spanish Super Cup in his first season.
The 23-year-old will remain at the Nou Camp until the end of the current campaign, which was delayed due to the coronavirus pandemic.
A statement on Barca's official website read: "FC Barcelona and Juventus FC have reached an agreement for the transfer of player Arthur Melo. The Italian club will pay 72 million euros, plus 10 million euros in variables.
"The player will remain at FC Barcelona until the end of official competition in the 2019-20 season."
Barcelona are two points adrift of leaders Real Madrid with six league games remaining and the Brazilian looks set to be available for the Spanish club when the Champions League resumes in August.
Buffon and Chiellini, meanwhile, will remain with the Serie A leaders, who are chasing a ninth consecutive Scudetto this season, until the end of the 2020-21 campaign.
Buffon, 42, joined Juventus from Parma in 2001 and has made over 500 appearances for the Bianconeri, returning to Turin at the start of this season following a one-year spell at Paris St Germain.
The World Cup-winning goalkeeper has played a back-up role to former Arsenal keeper Wojciech Szczesny, but will continue with the club until at least the end of next season.
Captain Chiellini, who has been at Juventus for 16 years, has also signed a new 12-month extension.
The 35-year-old centre-back joined from Livorno in 2004 and has made 509 appearances for the club in all competitions.