The Serie A powerhouse have named previous Italian national team head coach the veteran manager as their manager for the remainder of the campaign.
Spalletti takes over from his predecessor, who was let go post the Lazio setback last weekend.
The former boss came on board in March to replace Thiago Motta but left after seven months at the end of an eight-match winless run.
Spalletti led Napoli to the Serie A title in 2022-23 then moving on to lead the Italian national team, which missed out on the 2022 World Cup under former boss Roberto Mancini.
He secured their spot in Euro 2024 exiting in the last 16 stage against Switzerland.
Nonetheless, Italy dismissed Spalletti in June post a qualifying defeat against Norway, putting Italy in danger of missing a third straight 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 during the 3-1 victory over Udinese on Wednesday which left them seventh in Serie A - 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.
