The Portuguese Football Federation confirmed in a statement on its website that the superstar of the world game had tested positive for the virus on Monday.
The 35-year-old Juventus striker is “well, has no symptoms and is in isolation”, the federation said.
The news came just hours before Portugal’s Nations League match against Sweden on Wednesday in Lisbon.
Ronaldo, one of the most prolific goal scorers in the sport’s history, was dropped from the match as a result.
The federation said the rest of the Portugal squad had undergone tests as a result of Ronaldo’s positive result, but that they had all tested negative and would still play.
Ronaldo will now be doubtful for Juventus’ Serie A trip to Crotone on Saturday and their Champions League group stage opening game away to Dynamo Kiev next Tuesday.
And depending on his condition and the results of future tests, he could also miss the much-anticipated Champions League match against Barcelona on October 28.
Italy’s previous Serie A season was put on hiatus for months due to the pandemic and Juventus has seen a number of players test positive for coronavirus in the past.
The whole squad was put into isolation on October 4 when two club staff members tested positive for the virus.
As a result, there has been some controversy that Ronaldo and six other players were allowed to leave the isolated group and join their national teams.