The 35-year-old arrived at the Juventus Training Centre in a Jeep with tinted windows around 09:20 (0720 GMT), leaving three hours later with a smile and thumbs up for waiting photographers.
Ronaldo underwent medical and physical tests before joining up with his teammates for the first time in 72 days for individual training, according to media reports.
Coach Maurizio Sarri has been conducting training in small groups since Monday, pending the final health protocol to be agreed with the Italian government.
Juventus players got back to individual training on May 4, the day Ronaldo returned to Italy after spending lockdown in his native Portugal.
For the past two weeks, he has been in quarantine in his villa in Turin.
Ronaldo played in Juventus' last Serie A game before the season was suspended, a 2-0 win over Inter Milan behind closed doors at the Allianz Stadium on March 8.
The five-time Ballon d'Or winner is the first Juventus foreign player, among those who left Italy during the lockdown, to return to the team's Continassa training centre.
Italy has been one of the countries hardest hit by the COVID-19 pandemic with more than 32,000 deaths.
League leaders Juventus are leading Lazio by one point as they target a ninth consecutive 'Scudetto'.
The Italian Football Federation (FIGC) announced on Monday that all its competitions, including Serie A, will remain suspended until June 14.
Serie A had hoped to return on June 13 but the FIGC has pushed back the date in line with a government decree that all sports competitions be suspended until next month.
The FIGC said the choice was made "pending any further... decision by the authorities", suggesting the possibility of a restart on June 13 might still exist.
But Italian Prime Minister Giuseppe Conte said on Saturday he needed "more guarantees" before the green light could finally be given for football to resume.
A meeting is planned in the near future between Conte and Italian football bosses.
The government's technical and scientific committee will decide in the coming days if it accepts the new health protocols proposed for a return to group training and competition. -- AFP