The former playmaker failed to live up to expectations as a coach and Juventus brought an abrupt end to his reign.
The Bianconeri wasted little time in revealing that Allegri has returned as boss of the Serie A giants, who were dethroned as Italian champions by Inter this month.
Allegri, who was linked with Inter and Real Madrid before agreeing to rejoin his former club, had been out of work since ending his hugely successful tenure with Juventus two years ago.
The 53-year-old masterminded five consecutive Scudetto triumphs in his first spell at the club, and his team lifted the Coppa Italia on four occasions.
Juve were beaten Champions League finalists with Allegri at the helm in 2014-15 and 2016-17.
"Allegri finds a bench that he knows very well, a club that he loves and that loves him back, as today a new journey begins together, towards new goals.