The Old Lady have chosen ex- Azzurri manager the veteran manager as their manager until the end of the season.
The 66-year-old takes over from Igor Tudor, who was let go following the loss to Lazio on Sunday.
Tudor arrived at the club in March to replace Thiago Motta but departed after seven months following a string of eight matches without a victory.
Spalletti guided Napoli to the Serie A crown in 2022-23 prior to assuming the Italy role, after the team's failure to reach the 2022 World Cup under former boss Roberto Mancini.
He helped them qualify for Euro 2024 where they were knocked out in the last 16 by Switzerland.
However, Spalletti was sacked by Italy in June after a World Cup qualifying defeat by Norway, with the Azzurri at risk of missing out on a third consecutive World Cup.
Spalletti has also managed Inter Milan, Roma and Udinese in Serie A and won the Russian Premier League twice with Zenit St Petersburg.
Juventus were temporarily led by Massimo Brambilla in the midweek 3-1 triumph over Udinese placing them seventh in the Serie A standings - trailing leaders Napoli by six points after nine matches.
Their Champions League campaign sees them in 25th place in the group stage, with one loss and two draws in their first three games.
