The 25-year-old scored five goals - including the winner against the Serie A runners-up in April - in 37 league appearances in the 2018-19 season.
Di Lorenzo's move was made official on Friday with media reports in Italy suggesting he completed a medical last week.
"I'm happy about my new adventure at a prestigious club like Napoli," he told his new team's official website. "I can’t wait to start the new season and get to know my new team-mates."
Di Lorenzo - who represented Italy at under-21 level twice in 2013 - was unable to prevent Empoli from suffering relegation on the final day of the campaign.
The 25-year-old is an adaptable right-sided player who has been used in a advanced positions by Empoli as well as at his preferred full-back position.
He is a capable performer on the right wing and has also been employed as a central midfield player during his short career.
Napoli boss Carlo Ancelotti has made it known he intends to sign players who he believes will be adaptable to his team's style of play as he begins the job of masterminding closing the gap on runaway Serie A champions Juventus.
The manager will also be hoping for his side to give a stronger showing in the Champions League having failed to make it past the group stage last term.
The team finished 11 points behind Juve in Serie A, failing to sustain a serious title challenge or put pressure on the champions as they racked up a seventh successive league title.
Napoli were also knocked out of the Champions League before the knockout rounds, missing out in a tough group containing PSG and eventualy winners Liverpool.
They lost 1-0 at Anfield on the final matchday to be eliminated in third place.