Ron Johnson is the third GOP senator to announce that he has the coronavirus this week.
The Republican from Wisconsin said that he is not experiencing symptoms and will remain in isolation.
According to a statement on Saturday, Sen. Johnson was in quarantine for 14 days after he was exposed to someone with Covid-19 on 14 September. He tested negative for Covid-19 twice during that period.
He returned to DC on 29 September and was exposed to an individual who has since tested positive.
The senator, 65, was then tested on Friday and the results were positive.
The statement noted that the senator's office will go all-virtual for the time being and that most staff had been working remotely already.
In April, Mr Johnson sent a letter to the White House signed by nearly 800 physicians urging the president to take executive action eliminating federal restrictions on the drug hydroxychloroquine.
The president touted the effectiveness of the drug in treating coronavirus in the early weeks of the pandemic, but has since stopped talking up the drug after numerous studies have shown it to have little effect on patients.
In addition to Mr Johnson, Senators Mike Lee and Thom Tillis both confirmed on Friday that they have Covid-19.
The announcements came as President Trump was taken to Walter Reed Medical Center to be treated for coronavirus on Friday.
Both Mr and Ms Trump tested positive for the virus, and both have shown symptoms. All of Mr Trump's public events have been cancelled for the foreseeable future.
Senators scrambled to get tested Friday after the president's diagnosis.
In addition to the three senators who have tested positive, presidential aide Kellyanne Conway, another White House staffer, three journalists based in the White House, and the President of Notre Dame - who attended an event at the White House a week ago - have all tested positive for Covid-19.
Several of the individuals who have tested positive appear to have contracted the virus during a Supreme Court nomination ceremony to honour Ms Barrett in the Rose Garden on 26 September.
