White House lawyer Ty Cobb confirmed the move to CNBC.
“I’m doing it because I hope we can negotiate an end to this for the good of the country and because I have high regard for the president and for Bob Mueller,” Giuliani told The Washington Post.
Giuliani is a close ally of the president’s and was a top Trump supporter on the campaign trail.
Last month, John Dowd resigned as the president’s top outside attorney representing himin the investigation into Russian efforts to influence the 2016 election and whether the Trump campaign colluded with Moscow.
Giuliani previously served as the United States Attorney for the Southern District of New York. Trump’s long-time lawyer Michael Cohen is currently under investigation by the Southern District.
According to the New York Times, the probe of Cohen has become more worrisome for the White House than the special counsel’s investigation into Russian interference in the 2016 presidential election and alleged collusion by the Trump campaign.
Trump considered Giuliani for an administration post when he won the 2016 election but did not end up choosing him for a job.
Giuliani was a federal prosecutor in Manhattan in the 1980s. He was mayor of New York City during the Sept. 11, 2001, terror attacks.
Giuliani Partners, a security and management consulting firm founded by Giuliani, and representatives for Giuliani did not immediately respond to CNBC’s requests to comment.