The 81-year-old's condition is reported to be critical but stable. The reason for his hospitalization has not been disclosed.
Giuliani was the city's mayor from 1994 until the end of December 2001. The former prosecutor was praised for his actions against crime and for bringing New Yorkers together after the terrorist attacks of September 11, 2001.
He made unsuccessful bids for the White House in 2008 and 2012. In 2016, Giuliani began serving as an advisor to Donald Trump and later became the president's personal lawyer - including in matters related to alleged election fraud.
"Our amazing Rudy Giuliani, a true warrior and by far the best mayor of New York City ever, is being treated in the hospital," Trump wrote on Truth Social.
