The Serie A powerhouse have appointed ex- Italian national team manager the veteran manager to lead the team for the remainder of the campaign.
Spalletti replaces Igor Tudor, who was dismissed after the Serie A side's defeat by Lazio recently.
The former boss came on board in March to replace Thiago Motta but left after seven months following a string of eight matches without a victory.
Spalletti guided Napoli to the Serie A crown in 2022-23 then moving on to lead the Italian national team, which missed out on the 2022 World Cup during Roberto Mancini's tenure.
Under his guidance, they made it to Euro 2024 where they were knocked out in the last 16 by Switzerland.
Nonetheless, Italy dismissed Spalletti in June after a World Cup qualifying defeat by Norway, 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.
Massimo Brambilla served as interim manager for Juventus during the 3-1 victory over Udinese on Wednesday placing them seventh in the Serie A standings - six points adrift of Napoli at the top after nine rounds.
They are 25th in the league phase of the Champions League, with one loss and two draws in their first three games.
