The 26-year-old has agreed a five-year contract and will join his former Napoli boss Maurizio Sarri at Stamford Bridge, having previously been linked with a move to Premier League champions Manchester City.
Jorginho – who will wear the No 5 shirt at Chelsea – has eight Italy caps and made over 100 appearances for Napoli, who signed him at the start of 2014.
“I am absolutely ecstatic to be here at Chelsea,” he said.
“It is not easy to become part of such a big team so I am very, very happy. I am excited to play in such an intense league, for a team that gives everything to play and win.”
Club director Marina Granovskaia said: ‘We are delighted Jorginho has chosen to join Chelsea.
“He was one of the most coveted midfielders in Europe and will become an important member of the playing squad.
“He has demonstrated his quality working under Maurizio Sarri and we are convinced his signing will help bring further success to Stamford Bridge.”
Confirmation of Jorginho’s arrival came on Saturday morning, just a couple of hours after it was announced that Sarri would take charge on a three-year deal.