The warrant was in relation to an ongoing probe led by federal prosecutors from the Southern District of New York regarding Giuliani's alleged lobbying efforts abroad during the Trump presidency, which was a key focus of the first impeachment case against then-President Trump.
Giuliani was awoken by federal agents at 6 a.m. at his home on Manhattan's Upper East Side, his attorney, Bob Costello, told ABC News. Giuliani's electronic devices, including his cellphone, were confiscated by authorities.
At Giuliani's office, agents seized devices including a computer belonging to longtime Giuliani assistant Jo Ann Zafonte, Costello said.
Costello also told ABC News that Zafonte was served with a subpoena to appear before a federal grand jury next month.
"They're trying to make Rudy Giuliani look like a criminal," Costello said. "He has done nothing wrong."
Costello said he had previously offered to have Giuliani come in for an interview with federal prosecutors if they would provide a sense of what they were seeking, but that the offer was rejected.
Officials with the FBI and the U.S. Attorney for the Southern District of New York declined to comment.
Giuliani's work on matters related to Ukraine and his business dealings with two Soviet-born associates has been the subject of an ongoing investigation by prosecutors in the Southern District of New York, sources previously told ABC News.
The two businessmen, Lev Parnas and Igor Fruman, were arrested and indicted in October 2019 as part of what the government said was a "foreign national donor scheme" that allegedly sought to funnel foreign money to American political candidates.
Parnas and Fruman, along with two others also arrested, allegedly funneled "$1-2 million" from a Russian donor into the U.S. political system between June 2018 and April 2019, according to prosecutors. Both pleaded not guilty.
Parnas and Fruman reportedly assisted Giuliani in his efforts to dig up dirt on the then-Democratic rival of his main client, President Trump. That work eventually became an issue in Trump's first impeachment.
ABC News reported in December that federal prosecutors in New York were considering pursuing Giuliani's electronic communications as part of the probe, two sources familiar with the matter said at the time.
News of the search warrant was first reported by The New York Times.
In connection with the probe, federal agents also carried out a search warrant at the home and office of Washington-based attorney Victoria Toensing, who has been a close ally of Giuliani and an occasional adviser to Trump, sources told ABC News.
Agents did not search the premises but only took possession of Toensing's cellphone, a source said.
"Ms. Toensing is a former federal prosecutor and senior Justice Department official," said a statement released by Toensing's law firm, diGenova & Toensing. "She would have been happy to turn over any relevant documents. All they had to do was ask. Ms. Toensing was informed that she is not a target of the investigation."
In addition to Toensing, sources told ABC News that several other individuals were named in the search warrant served on Giuliani Wednesday morning, many of whom had ties to Ukraine, sources said.