Cristiano Ronaldo, one of soccer’s biggest stars and among the world’s most famous athletes, has tested positive for the coronavirus, Portugal’s soccer federation announced Tuesday.

The federation’s statement said Ronaldo was not displaying symptoms of Covid-19, and that he had entered isolation, away from the rest of Portugal’s players. Those players continued their preparations for a match against Sweden on Wednesday in Lisbon.

“Following the positive case, the remaining players underwent new tests Tuesday morning,” Portugal’s federation said in a statement. “All tested negative.”

Ronaldo, 35, will miss Wednesday’s Nations Cup game against Sweden, the federation said.

Ronaldo played for Portugal in a friendly against Spain last week and in a scoreless Nations League draw against France on Sunday. On Monday, he posted a photo of himself dining with his smiling teammates on his social media accounts.

“United on and off the field,” the caption read.

Ronaldo’s positive test confirmed the worst fear of some European clubs, who had fought with FIFA, soccer’s global governing body, to alter rules that require them to release players to represent their national teams during scheduled international breaks. Many clubs feared that sending their players on long trips to virus hot spots — particularly in South America, where qualifying for the 2022 World Cup began last week — could leave them vulnerable to infection or cause them to bring the virus back to their European teams.

Players who did travel now will face quarantines or other restrictions — and potentially even miss league games — upon their return to certain countries, including the United States.

Ronaldo, who plays for the Italian club Juventus, had been spared infection earlier this year when the virus swept through his team and the Italian league system. At least three Juventus players — Paulo Dybala, Daniele Rugani and Blaise Matuidi — tested positive in the first wave of cases earlier this year.

Ronaldo is not the first soccer star to test positive this fall as Europe’s top leagues start their new seasons and players journey home for national team duty. The Manchester United midfielder Paul Pogba was found to be positive when he turned up for a training camp with France’s national team in August, weeks after the Brazilian star Neymar and two of his teammates at the French club Paris St.-Germain are believed to have caught the virus during a postseason vacation in Spain.

On Monday, Peru’s soccer federation announced that two of its players would miss a qualifier against Brazil on Tuesday night in Lima after testing positive.

While many of Europe’s top leagues were able to resume their seasons after pausing play for several months during the height of the pandemic this spring, the coronavirus remains a significant threat because — unlike in the restricted environments set up for this summer’s Champions League knockout rounds or the recently completed N.B.A. season — players are free to circulate in their communities.

Liverpool, the reigning champion of England’s Premier League, has had several players test positive in recent weeks. Forward Xherdan Shaqiri was the most recent, though his test result was later reclassified as a false positive. It followed earlier cases involving Thiago Alcantara and Sadio Mané.

In the top league in the United States, Major League Soccer, several games have been postponed or rescheduled this year after players or staff members tested positive. On Monday, the league announced that it had postponed the next three matches involving the Colorado Rapids after an outbreak on that team.

M.L.S. said the team had not recorded any new cases, but that the decision to postpone the games “was made based on the total number of current cases and in consultation with the Colorado Department of Public Health and Environment.”