Beukema joined Bologna of AZ Alkmaar in 2023, and the 26-year-old made 80 performances in all matches for the Italian side in the past two seasons, including starting all eight games in their Champions League campaign last season.
Napoli, who won their fourth Scudetto in May, also added the Belgian midfielder Kevin De Bruyne, the Dutch winger Noa Lang and the Italian striker Lorenzo Lucca to the Antonio Conte team in the narrow season.