The spokesman, Ted Goodman, did not immediately provide more details about Giuliani's condition or how long he has been hospitalised.
"Mayor Giuliani is a fighter who has faced every challenge in his life with unwavering strength, and he's fighting with that same strength now," Goodman said in a statement on social media.
"We do ask that you join us in prayer for America's Mayor Rudy Giuliani."
Giuliani, 81, hosted his online show, America's Mayor Live on Friday night, US time, from Palm Beach in Florida.
As he opened the show, he coughed and his voice sounded more raspy than usual.
"My voice is a little under the weather, so I won't be able to speak as loudly as I usually do, but I'll get closer to the microphone," he said.
Giuliani garnered acclaim for his response to the September 11, 2001 al-Qaeda attack on New York City, earning the nickname "America's Mayor".
Giuliani worked as a lawyer for US President Donald Trump in his failed efforts to overturn the results of the 2020 election, an effort that led to criminal charges against Giuliani in two American states and a defamation lawsuit from election workers.
Giuliani denied wrongdoing in the criminal cases, and Trump pardoned him.
"Our fabulous Rudy Giuliani, a True Warrior, and the Best Mayor in the History of New York City, BY FAR, has been hospitalized, and is in critical condition," Trump wrote on his social media platform.
"What a tragedy that he was treated so badly by the Radical Left Lunatics, Democrats ALL — AND HE WAS RIGHT ABOUT EVERYTHING! They cheated on the Elections, fabricated hundreds of stories, did anything possible to destroy our Nation, and now, look at Rudy. So sad!"
However, two former election workers won a $US148 million ($A205 million) defamation judgment against Giuliani.
He ultimately struck a deal that let him keep his homes and various belongings in exchange for unspecified compensation and a promise to stop speaking ill of the ex-election workers.
Giuliani was elected New York's mayor in 1993 after serving as one of the nation's highest-profile prosecutors, taking on mobsters and crooked Wall Street traders.
He ran for the US Senate in 2000 but abandoned his race against Hillary Clinton after being diagnosed with prostate cancer.
Giuliani was previously hospitalised in 2025 after suffering a fractured vertebra and other injuries following a car crash.
With AP
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