The federation said Ronaldo was doing well and had no symptoms. It did not say when he tested positive.
Ronaldo played in the 0-0 draw at France in the Nations League on Sunday, and also in the 0-0 draw against Spain in a friendly last week.
Ronaldo is in isolation and has been dropped from the country's Nations League match against Sweden on Wednesday in Lisbon.
On Monday, the Juventus forward posted a photo on Twitter showing him and the rest of the Portugal squad having a meal together. The players were all close to each other at a table, with Ronaldo apparently taking the photo himself at the front end.
Along with the tweet, Ronaldo wrote, in Portuguese, “United on and off the field!”
The federation said Ronaldo's result prompted another round of tests for the rest of the Portugal squad. It said everyone else in the team tested negative.
Tuesday's practice session was moved from the morning to the afternoon so there was time for all the results to come back.
Two other Portugal players had tested positive for COVID-19 recently, José Fonte and Anthony Lopes.
Portugal coach Fernando Santos said last week the positive results among players were “happening everywhere in the world.”
“We are the most tested people and we have the guarantee that everyone who will be traveling to France with us are negative,” Santos said before the match against France.
Portugal and France lead Group 3 with seven points each after three matches.
The Portuguese federation did not immediately release details about where Ronaldo was in isolation, or for how long he would stay there.
He could miss Juventus’ Serie A match at Crotone on Saturday and the team’s Champions League opener at Dynamo Kyiv next Tuesday. And depending on his condition and the results of future tests, he could also miss the much-anticipated match against Barcelona on Oct. 28.
Juventus had been in isolation following positive tests for two staff members with the club. There has been some controversy that Ronaldo and six other players were allowed to leave the isolated group and join their national teams.
