The pair would have seen their previous deals expire this summer but will now stay until at least the end of the 2020/2021 campaign.
"These are players who need no introduction," a Juve statement read.
"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."
Chiellini has been at Juventus since 2005 and has won eight Serie A titles.
The 35-year-old Italian international has spent majority of the 2019/2020 season on the sidelines after suffering an anterior cruciate ligament damage to his right knee back in August that put him out of action for six months.
Chiellini returned to action in February but the Serie A season was suspended shortly after because of the coronavirus pandemic, which has restricted him to three appearances in all competitions this term.
42-year-old Buffon has spent all but one of the last 19 seasons with Juventus. He returned to the club last summer after spending the 2018/2019 season in France with Paris Saint Germain.
After agreeing the contract extension, Buffon said: "Probably each of us hoped, and basically knew, that the thread that united us for so many years was not destined to break."
Juventus are four points clear at the top of the Serie A table and could win their ninth straight league title this season.
