The central midfielder, who has impressed in Serie A this season, joins the Partenopei from Verona on a four-and-a-half year co-ownership deal
The midfielder, 22, had started 18 out of the newly promoted side's 19 Serie A matches so far this season and his impressive form saw him linked to the Partenopei in the winter window.
He told the club's official website: "I'm happy to wear the shirt of Napoli. They are a big team with a great crowd. It will be great to play at the San Paolo and I cannot wait to start my adventure in blue."
The San Paolo side's president Aurelio De Laurentiis confirmed the deal had been completed following his medical on Saturday and suggested his arrival will boost Rafa Benitez's men.
"Welcome Jorginho!" the Napoli chief wrote on Twitter. "Your freshness will be useful to Rafa and the whole squad."
The Italy Under-21 international has risen through the ranks from a youth prospect at Verona and the club wished him all the best after the confirmation of his move to Naples.
"Club president Maurizio Setti, coach Andrea Mandorlini, the rest of the coaching staff and former team-mates wish the best luck to Jorginho after what he has achieved in Gialloblu colours, and we hope he continues his wonderful career," the club said in a statement.