The former mayor of New York City was n temporary leave from the Greenberg Traurig firm when he joined Trump's legal team to handle matters related to special counsel Robert Mueller's probe last month. In a statement Thursday, Giuliani said the move would be permanent.
Mueller is investigating claims of Russian interference in the 2016 presidential election and whether Trump's campaign colluded in that effort.
"In light of the pressing demands of the Mueller investigation, I believe it is in everyone's best interest that I make it a permanent resignation," Giuliani said. "This way, my sole concentration can be on this critically important matter for our country."
Giuliani joined the team nearly one month after the resignation of John Dowd, a former personal attorney to Trump.
Greenberg Traurig's executive chairman Richard Rosenbaum said Giuliani's resignation went into effect Wednesday.
In a statement, he lauded Giuliani's career, including his leadership as New York's mayor in the 1990s and early 2000s.
"I have a great deal of respect for the mayor's incredible career and what he has done for New York City and our country for many years and consider him a friend," Rosenbaum said in a statement.
During his brief time working for Trump, Giuliani has revealed the president reimbursed his personal lawyer, Michael Cohen, for $130,000 in hush money regarding his alleged affair with porn star Stormy Daniels.
Though the reimbursement was made only a month before Trump's 2016 election, Giuliani has said it "was not campaign money," and therefore it was not a campaign finance violation.