Giuliani, who served as former President Donald Trump's personal lawyer, has been the focus of an investigation concerning his activities in Ukraine, including whether he conducted illegal lobbying for Ukrainian officials while he pursued an investigation linked to Trump's primary political rival, President Joe Biden, CNN has reported.
The raid was ordered as part of a probe into Mr Giuliani's dealings with Ukraine, officials say.
The effort to pressure Ukraine led to Mr Trump's first impeachment in 2019.
Executing a search warrant against a lawyer is a move not taken lightly.
Federal investigators on Wednesday searched the New York City apartment of former Mayor Rudolph Giuliani, who later became President Donald Trump's personal lawyer, seizing electronic devices as they stepped up a criminal investigation, the New York Times reported.
The federal probe into Giuliani's overseas and business dealings stalled a year ago because of a dispute over investigative tactics as Trump unsuccessfully sought reelection, and amid Giuliani's prominent role in subsequently disputing the results of the contest on Trump's behalf. Mr Giuliani had previously called the investigation "pure political persecution". His lawyer Robert Costello claimed the searches were "unnecessary" because Giuliani offered to answer questions from prosecutors.
He added that the raids stem from an "alleged incident of failure to register as a foreign agent".
That consulting work posed no risks when it was confined to foreign soil, but raising anything on behalf of foreign clients with USA officials could raise potential violations of the Foreign Agent Registration Act, which requires lobbyists to disclose when they are working on behalf of foreign governments, according to legal experts.
According to the news outlet, federal agents have been investigating Giuliani's potential involvement in pressuring Ukrainian officials to investigate the Bidens as well as his role in pushing for the ouster of then-U.S. ambassador to Ukraine, Marie Yovanovitch. Hunter Biden is facing an unrelated criminal tax probe from the US Justice Department.
Agents searched Giuliani's home on Madison Ave and his office on Park Ave, people familiar with the investigation told the Associated Press.
Investigators have released few details so far about the case and it remains unknown whether the Republican will eventually face any charges.
A federal warrant requires approval by a judge.
Giuliani also sought to undermine former USA ambassador to Ukraine Marie Yovanovitch, who was pushed out on Mr Trump's orders, and met several times with a Ukrainian lawmaker who released edited recordings of Biden in an effort to smear him before the election.
After Mr Trump's election loss in November, Mr Giuliani toured the country speaking to Republican state legislators in an effort to get state lawmakers to overturn the result in favour of Mr Trump.