Juventus have chosen ex- Italian national team boss the veteran manager to lead the team for the rest of the term.
Spalletti takes over from Igor Tudor, who was dismissed after the Serie A side's defeat by Lazio on Sunday.
Tudor had joined the club in March filling the shoes of Thiago Motta but left after seven months following a string of eight matches without a victory.
Under Spalletti, Napoli clinched the Serie A championship in 2022-23 before taking charge of Italy, after the team's failure to reach the 2022 World Cup under former boss Roberto Mancini.
He secured their spot in Euro 2024 exiting in the last 16 stage against Switzerland.
Yet, the Italian FA let Spalletti go in June post a qualifying defeat against Norway, with the Azzurri at risk of missing out on a third consecutive World Cup.
Spalletti's managerial career includes Inter Milan, Roma, and Udinese in Serie A and won the Russian Premier League twice with Zenit St Petersburg.
Massimo Brambilla took interim charge of Juventus for their 3-1 win against Udinese on Wednesday 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, having lost once and drawn twice in their opening three matches.
