The Portuguese superstar travelled back to Italy from Madeira two weeks ago, having spent time on his native island during the coronavirus crisis.
He spent the mandatory two weeks in quarantine before being cleared to head back to Juve's Continassa base in the north of Turin, where five-time Ballon d'Or winner Ronaldo arrived to undergo medical checks.
The 35-year-old was pictured arriving and departing the training centre, with Turin-based sports newspaper Tuttosport saying Ronaldo drove in at 09:21 local time before leaving around three hours later.
The former Real Madrid and Manchester United forward was pictured in his Jeep car, and he reportedly smiled and nodded to waiting journalists on his exit.
Juventus head coach Maurizio Sarri is said to be separating his squad into groups of six to seven players as team training resumes.
Many members of Sarri's squad have been training individually at the club headquarters since May 5.
Juventus sit top of Serie A, having beaten Inter 2-0 in their final match before the COVID-19 pandemic caused the league to be suspended. They hold a one-point lead over Lazio.
Italy's football authorities have yet to announce when the league will resume but have indicated a hope that matches can take place from mid-June.
There is no prospect of games before then, after the Italian Football Federation (FIGC) said on Monday that Serie A would remain suspended until at least June 14.
There have been over 225,000 confirmed cases of COVID-19 in Italy with more than 32,000 deaths.