The Spanish attacker has been announced as a San Paolo player for the season by club chief Aurelio De Laurentiis, ending his two-year spell in the Premier League
The 28-year-old was a revelation at the Liberty Stadium in his opening season in the Premier League, in 2012-13, but was plagued by injury last campaign and only started 15 times as a result.
The Spaniard has been strongly linked with a move to Serie A this summer - rumours which were solidified when he did not join the club on their pre-season tour of USA - and Swansea confirmed last week that Napoli were in talks over signing Michu.
De Laurentiis finally confirmed the loan deal had been completed on Thursday, with Napoli having an option to buy the ex-Rayo Vallecano star add the end of the season.
"Welcome to Michu," the Stadio San Paolo chief wrote on his Twitter, with his message being retweeted by the club's official account.
Michu will link up with coach Rafa Benitez, a fellow Spaniard, at Napoli and joins fellow compatriots Jose Callejon and Raul Albiol at the Serie A giants, who finished third in the Scudetto race last season.