Despite boasting no managerial experience, the iconic ex-midfielder was hired on Saturday, just hours after the club decided to fire Maurizio Sarri.
The move, which came out of left field, left Twitter in shock. Here are some of the best reactions to the stunning appointment:
Juve goalkeeper Gianluigi Buffon, a former teammate of Pirlo's during the latter's four-year spell at the club, wished his compatriot luck, but cheekily asked whether he now has to call him "coach."
Buffon is one year older, after all.
Pirlo's affinity for wine was also a hot topic after his hiring became official ...
... as was his lack of coaching experience.
As always, the Twitterverse also had its share of jokes: