Lozano spent two seasons in the Eredivisie, scoring 34 league goals for PSV across the two campaigns, and impressed in the Champions League last term.
The 24-year-old winger also starred for Mexico at the 2018 World Cup, scoring a memorable goal in a 1-0 win over Germany in the group stage.
Lozano, who had also been linked with Manchester United, has now swapped the Eredivisie for Serie A, with Napoli reportedly paying a club-record transfer fee reported to be €38m plus bonuses.
Prior to the club's official announcement on Friday, Napoli president Aurelio De Laurentiis had shared a photo of Lozano signing his contract on his official Twitter account.
Napoli kick off their Serie A campaign against Fiorentina on Saturday.