The Old Lady have chosen former Italian national team boss the veteran manager as their manager for the rest of the term.
Spalletti replaces Igor Tudor, who was dismissed following the loss to Lazio recently.
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 guided Napoli to the Serie A crown in 2022-23 prior to assuming the Italy role, which missed out on the 2022 World Cup with Mancini at the helm.
He helped them qualify for 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 has also managed Inter Milan, Roma and Udinese in Serie A and lifted the Russian league trophy twice while at Zenit St Petersburg.
Massimo Brambilla served as interim manager for Juventus during the 3-1 victory over Udinese on Wednesday moving them to seventh position in the league - trailing leaders Napoli by six points after nine matches.
They are 25th in the league phase of the Champions League, after recording one defeat and two ties in the initial three fixtures.
