"I hope that my fantastic story with this club will go on and bring even further success," Buffon told a news conference. "I'm immensely proud in the faith Juventus have placed in me once more."
Barzagli added: "Even at this age, I still have room to improve and this is the best place for me to do so."
Buffon indicated earlier this year that he planned to retire after the 2018 World Cup, which would be his sixth. He and Barzagli helped Italy win the 2006 World Cup.
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Buffon joined Juventus in 2001 and became captain in 2012 following Italy great Alessandro Del Piero's move to Sydney FC.
He has made 458 appearances for the club, known as 'The Old Lady' and won nine league titles, including two that were stripped from Juve over the Calciopoli corruption scandal.
Buffon set a Serie A record of 974 minutes without conceding a goal as Juve won 4-1 against local rivals Torino on March 20.
The much-travelled Barzagli has made 150 appearances for the club after joining from Vfl Wolfsburg in 2011.
On Saturday, Juventus face 15th-placed Sampdoria in their last league game this season before being crowned champions.
(Writing by Silvia Recchimuzzi in Gdynia; Editing by Ken Ferris)
