The ltalian champions also confirmed defender Giorgio Chiellini has extended his deal at the club for another season.
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"There are players who need no introduction," the club said in a statement. "Champions whose story speaks for itself, and whose connection with the shirt they wear is indissoluble.
"Examples on the pitch and in the locker room, leaders, driving forces, bearers of the Bianconeri DNA, who wear it as a second skin.
"Gianluigi Buffon. Giorgio Chiellini. Captains, legends. Bianconeri since the beginning. Bianconeri forever. And to confirm it, in case there is a need, today their contract renewals have become official, for another: 2021!"
Buffon returned to Turin at the start of the 2019-20 campaign after a 12-month spell with Paris Saint-Germain where he won the Ligue 1 title and Trophee des Champions.
The former Italy international had previously spent 17 years with the Bianconeri where he won 12 Serie A titles -- though two were later taken away following the Calciopoli scandal -- and the Coppa Italia on four occasions.
Buffon has only made 13 appearances in all competitions this season and featured in the Coppa Italia defeat to Napoli earlier this month.
Chiellini has played for Juve since 2005 and made more than 500 appearances for the club and won eight league titles in the process.
Both players are looking to help Juve clinch a ninth consecutive Serie A title and their first Champions League trophy since 1996.