The Old Lady have named former Azzurri head coach the veteran manager to take the helm until the end of the season.
Spalletti replaces Igor Tudor, who was sacked after the Serie A side's defeat by Lazio last weekend.
Tudor arrived at the club in March as Thiago Motta's replacement but departed after seven months at the end of an eight-match winless run.
Spalletti led Napoli to the Serie A title in 2022-23 before taking charge of Italy, who had failed to qualify for the 2022 World Cup under former boss Roberto Mancini.
He helped them qualify for Euro 2024 exiting in the last 16 stage against Switzerland.
However, Spalletti was sacked by Italy in June following a loss to Norway in World Cup qualifying, putting Italy in danger of missing a third straight World Cup.
He has previously coached Inter Milan, Roma, and Udinese in Italy's top flight and secured two Russian Premier League titles with Zenit St Petersburg.
Juventus were temporarily led by Massimo Brambilla during the 3-1 victory over Udinese on Wednesday placing them seventh in the Serie A standings - six points behind leaders Napoli after nine games.
Their Champions League campaign sees them in 25th place in the group stage, after recording one defeat and two ties in the initial three fixtures.
