Swansea have announced that Michu has joined Napoli on a season-long loan deal with the Italian club holding an option to make the transfer permanent.
The 28-year-old, who arrived at the Liberty Stadium in July 2012 from Rayo Vallecano, completed a medical with Rafael Benitez's side on Thursday morning.
Michu scored 22 goals in all competitions during his first season at Swansea but struggled to recapture that impressive form last term as injuries took their toll.
It is understood Swansea were keen to agree a permanent transfer for Michu, who managed only two goals in 15 Premier League starts in 2013/14.
Napoli president Aurelio De Laurentiis tweeted a message of welcome to Michu on Thursday afternoon.
De Laurentiis wrote: "Benvenuto a Michu" - "Welcome Michu".
The physical centre forward proved an instant hit after moving to the Liberty Stadium, scoring twice on his league debut in a 5-0 victory over QPR.
An excellent campaign resulted in Michu being voted Swansea player of the year by both his team-mates and the club's supporters in May 2013.
The striker was called up to the Spain squad for their World Cup qualifier against Belarus in October but ankle and knee problems hampered his contribution for club and country.
Overall, Michu has played 63 games for the Swans and scored 28 goals.
Napoli will play in the Champions League this season after finishing third in Serie A.
Benitez's side also lifted the Coppa Italia for the fifth time in the club's history and knocked Swansea out of the Europa League in their last 32 tie in February.