Images in Italian media showed Ronaldo's private plane landing late Monday and he is expected to undergo 14 days of quarantine.
The Portugal international has been in Madeira with his girlfriend and children for the entirety of the lockdown in Italy due to the coronavirus pandemic.
He last featured for Juventus in a 2-0 league win over Inter Milan.
Football teams can resume training in Italy on May 18 while players can do individual workouts in the club's facilities starting this week.
A group of Juventus players were reportedly tested for COVID-19 on Monday at the J Medical in Turin, with the remainder of the squad expected to be tested once they arrive in Italy.