Allegri, 48, took charge in July 2014 and signed a new deal until 2017 after leading the Old Lady to the league title and the Coppa Italia.
Having successfully defended their league crown, Juventus have announced the former AC Milan boss will now remain until 30 June 2018.
Juventus face AC Milan in the Coppa Italia final on 21 May.
They are 12 points clear of nearest rivals Napoli with two games remaining in Serie A.
Juventus defeated Manchester City twice in the group stage of this season's Champions League before being knocked out by Bayern Munich in the round of 16.
Juventus boss Massimiliano Allegri has extended his contract with the Serie A champions until the summer of 2018.