The Serie A champions were looking for a new trainer following the shock departure of Antonio Conte on Tuesday and they have now named the former AC Milan coach as their new boss.
"For me it is an honour to be called as coach of Juventus," he told reporters at a press conference to reveal his arrival on Wednesday. "I am content and happy to coach Juventus.
"Today and tomorrow I begin my work. I am at the club's disposal. I know this is an important role. Juventus have won the Scudetto for three straight years, I am here to continue to winning with this team.
"I am lucky because for four years I had the fortune to coach Milan, and now I have the opportunity to coach another big team like Juventus. It is an honour and I want to keep winning.
"We will fight for the Scudetto until the bitter end. In Europe we have a duty to put in a big showing in the Champions League."
Allegri had been out of work ever since he was fired by the San Siro side back in January and he will now be looking to redeem himself at Juve.
The 46-year-old started his coaching career at Aglianese and also took charge of Real SPAL, Grosseto, Sassuolo and Cagliari before taking over at Milan in 2010.
He guided the San Siro outfit to the Serie A title in his first season, but failed to replicate that feat in the following seasons - partially due to the departure of stars such as Thiago Silva and Zlatan Ibrahimovic.
Allegri will be looking to guide Juventus - who confirmed the deal on their Twitter - to their fourth consecutive Scudetto, while the club's board of directors will also expect European success.
Juventus were disappointingly eliminated in the group stages of the Champions League last term after making it to the quarter-finals the season before.