Giuliani was exhibiting some symptoms and was admitted Sunday to Georgetown University Medical Center in Washington, according to a person familiar with the matter who was not authorized to speak publicly.
The 76-year-old former New York mayor has traveled extensively to battleground states in an effort to help Trump subvert his election loss. On numerous occasions he has met with officials for hours at a time without wearing a mask.
Trump, who announced Giuliani's positive test in a Sunday afternoon tweet, wished him a speedy recovery.
"Get better soon Rudy, we will carry on!!!" Trump wrote.
.@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!!!— Donald J. Trump (@realDonaldTrump) December 6, 2020
Giuliani did not immediately respond to a request for comment, but on Sunday evening he retweeted Trump's announcement of his diagnosis and then tweeted, "I'm getting great care and feeling good. Recovering quickly and keeping up with everything."
Thank you to all my friends and followers for all the prayers and kind wishes.— Rudy W. Giuliani (@RudyGiuliani) December 7, 2020
I’m getting great care and feeling good.
Recovering quickly and keeping up with everything.
He also tweeted thanks to a conservative writer who had said he was praying for Giuliani.
Giuliani attended a hearing at the Georgia Capitol on Thursday where he went without a mask for several hours. Several state senators, all Republicans, also did not wear masks at the hearing.
On Wednesday night, Giuliani was in Lansing, Michigan, to testify in a highly unusual 4 1/2-hour legislative hearing in which he pushed Republican lawmakers to ignore the certification of Joe Biden's Michigan victory and appoint electors for Trump. He did not wear a mask, nor did lawyer Jenna Ellis, who was sitting next to him. At one point, he asked one of his witnesses - a Detroit election worker - if she would be comfortable removing her mask. But legislators said they could hear her.
Giuliani traveled on Monday to Phoenix where he met with Republican legislators for an hourslong hearing in which he was maskless. The Arizona Republican Party tweeted a photo of Giuliani and several state GOP lawmakers standing shoulder-to-shoulder and maskless. The Arizona legislature announced Sunday, after Giuliani's diagnosis became public, that it would close for a week out of an abundance of caution "for recent cases and concerns relating to COVID-19."
The Trump campaign said in a statement that Giuliani tested negative twice before his visits to Arizona, Michigan and Georgia. Unidentified Trump team members who had close contact with Giuliani are in self-isolation.
"The Mayor did not experience any symptoms or test positive for COVID-19 until more than 48 hours after his return," according to the statement. "No legislators in any state or members of the press are on the contact tracing list, under current CDC Guidelines."
Giuliani also appeared maskless at a Nov. 25 hearing in Pennsylvania. And he did not quarantine after being near an infected person at a Nov. 19 news conference at the Republican National Committee's headquarters. His son Andrew Giuliani, who is a White House aide, announced a day after the event that he had tested positive for the virus.
Research shows that people who contract the virus may become infectious to others several days before they start to feel ill.
Georgia state Sen. Jen Jordan, a Democrat who attended Thursday's hearing, expressed outrage after learning of Giuliani's diagnosis.
"Little did I know that most credible death threat that I encountered last week was Trump's own lawyer," Jordan tweeted. "Giuliani - maskless, in packed hearing room for 7 hours. To say I am livid would be too kind."
Before the hearing, Giuliani and Michigan Republican Party Chairman Laura Cox - both maskless - did a virtual briefing for GOP activists.
Giuliani made an appearance earlier Sunday on Fox News "Sunday Morning Futures" to speak about his legal challenges in several states on behalf of Trump.
The diagnosis comes more than a month after Trump lost reelection and 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. The president's wife, Melania Trump, and teenage son, Barron Trump, have also been stricken.
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.
Those infected also include the White House press secretary and advisers Hope Hicks and Stephen Miller, as well as Trump's campaign manager and the chair of the Republican National Committee.
Trump spent the waning days of his campaign trying to persuade the American public that the virus was receding, and repeatedly claimed it would miraculously "disappear" after Nov. 3. Instead, the country is experiencing a record-breaking spike in infections.
The president gave the mounting coronavirus death toll scant attention at a Saturday evening rally in Valdosta, Georgia, where he campaigned on behalf of Republican Sens. Kelly Loeffler and David Perdue ahead of next month's runoff election there. Most people who attended the outdoor rally did not wear masks.
White House coronavirus task force coordinator Dr. Deborah Birx on Sunday offered tacit criticism of Trump's attitude on the virus during an interview on NBC's "Meet the Press."
Asked about Trump and other administration members flouting public health experts' warnings to avoid large gatherings and calls to wear masks, Birx replied that some leaders are "parroting" myths and called the pandemic "the worst event that this country will face."
"And I think our job is to constantly say those are myths, they are wrong and you can see the evidence-base," Birx added.
Lemire reported from Wilmington, Delaware. Associated Press writers Ben Nadler in Atlanta and David Eggert in Lansing, Michigan, contributed reporting.