An attorney disciplinary committee had asked the court to suspend Giuliani's license on grounds that he'd violated professional conduct rules as he promoted theories that the election was ‘stolen’.
The court agreed and said suspension should be immediate, even though disciplinary proceedings aren't yet complete, because there was an "immediate threat" to the public.
"The seriousness of respondent's uncontroverted misconduct cannot be overstated," the court wrote.
“This country is being torn apart by continued attacks on the legitimacy of the 2020 election and of our current president, Joseph R. Biden."
Trump called the suspension a politically motivated "witch hunt," while Giuliani said it was a "disgrace" on his afternoon radio show.
The court's opinion, Giuliani said, was based on hearsay and "could have been written by the Democratic National Committee."
"The bar association should give me an award," the Republican told listeners on WABC-AM.
"I defended an unpopular client. I've been threatened with death. I've had a good deal of my income taken away. I've lost friends over it."
"This is happening to shut me up," he added. "They want Giuliani quiet."
The court held that Giuliani, as a lawyer for Trump, "communicated demonstrably false and misleading statements to courts, lawmakers and the public at large."
Giuliani, a former New York City mayor and US attorney in Manhattan, claimed the investigation violated his First Amendment right to free speech and that he did not knowingly make false statements.
The court rejected those arguments, noting that in Pennsylvania, Giuliani failed to "provide a scintilla of evidence for any of the varying and wildly inconsistent numbers of dead people he factually represented voted in Philadelphia during the 2020 presidential election."
"False statements intended to foment a loss of confidence in our elections and resulting loss of confidence in government generally damage the proper functioning of a free society," the court wrote.
Giuliani will be allowed to fight the suspension and even call witnesses as part of his challenge — a process that could take months to play out — and Giuliani's attorneys said they expect him to be reinstated "once the issues are fully explored at a hearing."