The Old Lady have named ex- Italy boss Spalletti to lead the team for the rest of the term.
The 66-year-old replaces Igor Tudor, who was let go following the loss to Lazio last weekend.
Tudor had joined the club in March as Thiago Motta's replacement but departed after seven months following a string of eight matches without a victory.
Under Spalletti, Napoli clinched the Serie A championship in 2022-23 then moving on to lead the Italian national team, who had failed to qualify for the 2022 World Cup during Roberto Mancini's tenure.
He helped them qualify for Euro 2024 exiting in the last 16 stage against Switzerland.
Nonetheless, Italy dismissed Spalletti in June after a World Cup qualifying defeat by Norway, as Italy faced the possibility of not making a third World Cup in a row.
He has previously coached Inter Milan, Roma, and Udinese in Italy's top flight and lifted the Russian league trophy twice while at Zenit St Petersburg.
Massimo Brambilla served as interim manager for Juventus in the midweek 3-1 triumph over Udinese placing them seventh in the Serie A standings - six points behind leaders Napoli after nine games.
In the Champions League, they sit 25th in the league phase, with one loss and two draws in their first three games.
