"The Portuguese international is doing well, without symptoms, and in isolation," Portugal's Football Federation said Wednesday in a statement.
Ronaldo was to join Portugal for an international match against Sweden in the UEFA Nations League on Wednesday in Lisbon, but now will miss the game.
All other players on the Portugal national team were tested Tuesday after Ronaldo received the positive result. No other players tested positive, and they were cleared to train Tuesday afternoon.
The 35-year-old striker also played for Portugal during a 0-0 draw with France in the Nations League on Sunday in Saint-Denis, France. Ronaldo posted a photo while eating a meal with more than 20 of his Portugal teammates on Monday on Instagram. He also traveled on the team plane on their latest flight between games.
Ronaldo's status is in doubt for Italian club Juventus' game against Crotone in Serie A on Saturday in Crotone. Juventus also is scheduled to face Dynamo Kyiv in their first 2020-2021 Champions League game Oct. 20.
Ronaldo and Juventus have another Champions League match scheduled against Lionel Messi and Barcelona on Oct. 28.