The 35-year-old is doing well and not experiencing symptoms but will miss Portugal's Nations League match against Sweden on Wednesday.
All other players on the national team returned negative results.
Ronaldo took part in Portugal's last two games against Spain and France and posted a photo on Twitter of himself dining with the team.
"United on and off the pitch!" the caption read.
Ronaldo must now serve 10 days in quarantine and return a negative test result to resume play in Serie A with Juventus. He's likely to sit out his club's next two matches - Saturday against Crotone and next Tuesday against Dynamo Kyiv in the Champions League - and is at risk of missing the marquee clash against Lionel Messi's Barcelona on Oct. 28.
Ronaldo is the latest high-profile footballer to contract COVID-19. Liverpool star Sadio Mane is currently isolating after displaying minor symptoms, while AC Milan striker Zlatan Ibrahimovic has recently recovered from the virus.