Natali has put pen to paper on a contract running until the summer of 2031, with Leverkusen receiving a fee reported at between €2m and €3m plus bonuses, along with a 25 per cent sell-on clause should AZ move him on in future.
The son of former Italy defender Cesare Natali, he progressed through the academies of AC Milan, Espanyol and Barcelona before joining Bayer Leverkusen on a free transfer in the summer of 2024, having started for the Italy side that won the Under-17 European Championship that year while conceding just two goals across the tournament.
Napoli and Cagliari were among the Serie A clubs to have registered interest in taking him back to Italy, while Brighton were also credited with tracking his progress, but Natali opted to remain in the Netherlands.
During his season on loan, Natali featured only for AZ's youth and reserve sides, including appearances in the UEFA Youth League, rather than making the step up to first-team football.
"I'm delighted to be back here and to now finally play for AZ Alkmaar for good," Natali said.
"Last year I was welcomed so warmly by the team, the entire coaching staff and everyone at the club, so it's great to be able to work with everyone again."
The move continues a pattern of Leverkusen developing young talent before moving them on for a profit, following the recent permanent departure of forward Alejo Sarco. Natali will now be expected to push for involvement with AZ's senior squad after a year spent adapting to Dutch football away from the spotlight of first-team competition, having previously impressed enough during his loan to convince the Eredivisie club to make the move permanent rather than risk losing him to one of the Serie A sides monitoring the situation.
At 18, he is considered one of the more promising defensive prospects to have come through Barcelona's youth pathway in recent years, and the length of his new contract reflects AZ's confidence that he can eventually establish himself at the top level.