Fauci, 81, has mild symptoms and has been raised twice, the institute said in a statement. The institute told CNN that it is being treated with the antiviral drug Paxlovid.
NIAID said Fauci found it to be positive in a rapid antigen test. The director will work from home and follow the protocol of the agency, returning to the campus of the National Institutes of Health only after giving a negative.
Fauci has not had any close contact with Biden or other senior officials recently, the institute said.
Throughout the pandemic, he has called for caution and has shown himself in public. For example, he decided not to attend the annual White House correspondent dinner in April after considering his individual risk.
“Each of us, in our own personal way, has to do a risk assessment that we are willing to accept in order to become infected,” he told CNN. “In general, the risk is low, but I have made a personal assessment. I am 81 years old, and if I become infected, I have a much higher risk.”
Fauci has been working non-stop during the pandemic. In May, he said he had not had a day off since Covid-19 arrived in the United States.
He has been the director of NIAID since 1984 and has advised seven presidents.
Fauci joins a growing list of Biden administration leaders who have had Covid-19, including White House domestic policy adviser Susan Rice, Attorney General Merrick Garland, Vice President Kamala Harris and Secretary of Commerce Gina Raimondo.
CNN’s Nadia Kounang contributed to this report.