"[Rudy Giuliani], 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!" Trump, who also tested positive for coronavirus virus and was hospitalized in October, wrote on Twitter on Sunday afternoon.
Sunday night, Giuliani tweeted that he was "getting great care and feeling good."
"Recovering quickly and keeping up with everything," he said.
Giuliani has participated in various legal challenges to the results of November's election in key states such as Pennsylvania, Michigan and Arizona, which saw President-elect Joe Biden defeat Trump.
On Wednesday, Giuliani appeared without a mask at a hearing in the Michigan House of Representatives and on Thursday attended a hearing at the Georgia Senate Judiciary Committee contesting election results in that state.
Giuliani's son, Andrew Giuliani -- a special assistant to the president -- tested positive for COVID-19 on Nov. 20. Trump's son, Donald Trump Jr., also announced he had tested positive on the same day.