The Portuguese star is current with his national team for Nations League duties, but has now left the squad to head home and self-isolate after receiving news of his positive test.
He'll now miss this Wednesday's fixture against Sweden.
The Portuguese Football Federation confirmed the news in a statement which read:
"Cristiano Ronaldo was released from the work of the National Team after a positive test for COVID-19, so he will not face Sweden.
"The Portuguese international is doing well, without symptoms, and in isolation.
"Following the positive case, the remaining players underwent new tests this Tuesday morning, all with a negative result, and are available to Fernando Santos for training this afternoon, in Cidade do Futebol.
"The game, counting for the qualification phase of the League of Nations, is scheduled for Wednesday, at 19:45, in Alvalade."
Ronaldo will now miss at least two matches for Juventus, too, with Bianconeri set to take on Crotone this weekend before facing Dynamo Kyiv in the Champions League.
But he is also a doubt for the matches against Verona at home and, crucially, Barcelona in the Champions League. This is because Zlatan Ibrahimovic's positive test took 15 days to subside, and a repeat would put Ronaldo in serious danger of missing out on the visit of the Spanish giants on October 28.