The Old Lady have named previous Italy head coach the veteran manager as their manager for the rest of the term.
The 66-year-old takes over from the Croatian manager, who was sacked following the loss to Lazio recently.
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.
Spalletti led Napoli to the Serie A title in 2022-23 prior to assuming the Italy role, which missed out on the 2022 World Cup under former boss Roberto Mancini.
Under his guidance, they made it to Euro 2024 but were eliminated in the round of 16 by Switzerland.
Yet, the Italian FA let Spalletti go in June following a loss to Norway in World Cup qualifying, as Italy faced the possibility of not making a third World Cup in a row.
Spalletti's managerial career includes Inter Milan, Roma, and Udinese in Serie A and lifted the Russian league trophy twice while at 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 adrift of Napoli at the top after nine rounds.
Their Champions League campaign sees them in 25th place in the group stage, with one loss and two draws in their first three games.
