Maurizio Sarri says he wants to bring "entertaining football" to Chelsea after becoming their manager on a three-year deal.
The 59-year-old replaces fellow Italian Antonio Conte, who was sacked on Friday after two years at Stamford Bridge.
Sarri led Napoli to two second-place finishes in his three seasons in charge of the Serie A side, who named Carlo Ancelotti as his successor in May.
"It is an exciting new period in my career," said Sarri.
"I hope we can provide some entertaining football for our fans and that we will be competing for trophies at the end of the season, which is what this club deserves."
He is the 13th managerial appointment by Blues owner Roman Abramovich since his takeover of the club in 2003, including Jose Mourinho and Guus Hiddink on two occasions.
Sarri's appointment had been widely expected, with Chelsea agreeing compensation to bring him to Stamford Bridge because he had two years left on his Napoli deal and had remained on the Italian club's payroll after Ancelotti's arrival.
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Who is Sarri?
Italian football expert James Horncastle
Sarri never made it as a player and, unable to make a living as a footballer, he worked as a foreign currency trader at the Banca Toscana.
He was attached to the international department, which involved business trips to Europe's financial centres, including the City of London - but the role never stoked his imagination quite like football did.
He coached part-time, hopping from one Tuscan town to another and in 2001, when foreign currency traders like him were no longer as useful to banks as Italy prepared to adopt the euro, Sarri decided to take a leap of faith and leave his well-paid, regular-hours role, for what he says is "the only job I would do for free".
What did he do at Napoli?
Juventus have dominated the Italian league - winning seven titles in a row - but no side has come closer to them in that time than Napoli, who finished four points behind their rivals last season.
Sarri comes with a reputation as an attacking manager, with Napoli's haul of 94 goals in the 2016-17 league campaign remaining the highest number scored in a Serie A season since the turn of the century.
In his three years in charge, Napoli scored 251 league goals - more than any other side in the Italian top flight.
He is Chelsea's sixth Italian manager since Gianluca Vialli in 1998, following Claudio Ranieri, Ancelotti, Roberto di Matteo and Conte.
Despite his success domestically, he failed to guide the Naples club past the last-16 stage of the Champions League, finishing third in their group last season behind Manchester City and Shakhtar Donetsk.
He joined Napoli in 2015 after guiding Empoli to the top flight. Chelsea is his first managerial job outside his home country and he has yet to win a major trophy.