The 76-year-old personal lawyer to the president was admitted to Georgetown Hospital in Washington on Sunday. It is unclear when exactly Giuliani tested positive or the details of his condition.
Late Sunday, Giuliani posted on Twitter saying he was "feeling good."
President Donald Trump tweeted the news earlier Sunday and wished Giuliani a speedy recovery.
".@RudyGiuliani, by far the greatest mayor in the history of NYC, and who has been working tirelessly exposing the most corrupt election (by far!) in the history of the USA, has tested positive for the China Virus. Get better soon Rudy, we will carry on!!!" the president tweeted Sunday, repeating baseless claims of election fraud.
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Giuliani continues to lead Trump's legal challenges in an attempt to block Joe Biden's victory, despite mounting losses in court. Attorney General William Barr even told the Associated Press that the Justice Department uncovered no widespread fraud.
As recently as Thursday, Giuliani was at the Georgia state capitol, where he asked lawmakers to overturn the state's election results and select their own electors. Video shows Giuliani entering the capitol without a mask and high-fiving a supporter.
On Wednesday, he appeared at a four-hour Michigan legislative hearing to argue that fraud had occurred.
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The diagnosis comes more than two months after Trump himself was stricken with the virus in early October. Since then, a flurry of administration officials and others in Trump's orbit have also been sickened, including White House chief of staff Mark Meadows and Ben Carson, the secretary of housing and urban development.
Last month, Giuliani's son, Andrew Giuliani, a special assistant to Trump, said he tested positive for coronavirus.
On Twitter Sunday, Andrew said his father was "feeling well."
The extraordinary spread in Trump's orbit underscores the cavalier approach the president has taken to a virus that has now killed more than 280,000 people in the U.S. alone.
The Associated Press contributed to this report.