LAS VEGAS—President Biden has tested positive for Covid-19 , White House press secretary Karine Jean-Pierre said in a statement Wednesday.

Biden, 81 years old, will be returning to Delaware to self-isolate, Jean-Pierre said. He tested positive for Covid-19 during a two-day campaign swing through the battleground state of Nevada.

A note from Biden’s doctor provided by the White House said the president’s “symptoms remain mild, his respiratory rate is normal at 16, his temperature is normal at 97.8 and his pulse oximetry is normal at 97%.”

Biden has received his first dose of Paxlovid and the results of a PCR test are pending, according to the note. He previously had Covid in the summer of 2022.

Biden’s symptoms include a runny nose, cough and malaise, the note said.

Asked by reporters how he was feeling as he walked to Air Force One after news of the positive test, Biden, who wasn’t wearing a mask, responded: “I feel good.”

Biden has faced increasing pressure from Democrats in recent weeks, since his rough debate against Donald Trump , to consider ending his re-election bid . He has faced questions about his age and whether he has the stamina to endure the rest of the campaign and, if he wins, another four years in the White House.

The news of the president testing positive was first shared by Janet Murguía , president and CEO of UnidosUS, a Hispanic advocacy group, at their conference in Las Vegas, where Biden was scheduled to speak. Murguía said the president called her to deliver the news.

“He said to tell my folks that we’re not going to get rid of him that quickly,” Murguía said. “We’re going to have a chance to hear from him in the future directly. He’s just really sorry he couldn’t be with us. And obviously, we share that disappointment.”

Attendees at the convention had been waiting for Biden to speak for 90 minutes.

Jean-Pierre said Biden was tested after his first event in Las Vegas. Biden was taping a radio interview with Univision earlier on Wednesday.

Biden had just returned to the campaign trail on Tuesday for the first time since the assassination attempt on Trump on Saturday. He addressed the NAACP convention on Tuesday, stopped at a campaign event and at a local market, where he picked up dinner and met with employees.

Trump is accepting the Republican nomination in Milwaukee this week.

Dr. Lisa Sanders, medical director of Yale’s Long Covid Multidisciplinary Care Center, said Biden has a better chance than many other 81-year-olds of handling the disease well, because he is active.

Still, she said, “It’s going to be tough no matter who it is at that age.”

Sanders said the versions of Covid that are circulating now—the FLiRT variants —are a “kinder, gentler version” than what was around years ago. With the latest variants, people are falling into two groups. Some get respiratory symptoms that feel like a “bad cold,” she said. There is also the gastrointestinal version where people get abdominal pains, nausea and sometimes vomiting.

“I hope that he has a milder version and he’s back up and at it within a week or so,” she said.

Older age is a risk factor for severe Covid infection, which can include going into respiratory failure or developing pneumonia. “Even though this is a kinder, gentler version, people are still dying of this,” she said.

Sanders said people often feel better after a couple of doses of Paxlovid although they still need to take the whole five-day course.

“That might be a waste of breath for the president, but I think that people should rest when they have Covid,” she said. “The fatigue associated with Covid is really profound, and I don’t think it helps to try to push through it.”

Write to Tarini Parti at tarini.parti@wsj.com and Kayla Yup at kayla.yup@wsj.com