The central midfielder, who has impressed in Serie A this season, joins the Partenopei from Hellas Verona on a four-and-a-half year co-ownership deal
The midfielder, 22, had started 18 out of the 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.
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."