Giuliani said his participation in Trump's rally just before the Capitol riot prevents him from representing the president.
"Because I gave an earlier speech, I am a witness and therefore unable to participate in court or in the Senate chamber," Giuliani told ABC News on Sunday.
An unnamed source also told The New York Times that Giuliani wouldn't be Trump's lawyer in the proceedings.
Trump is accused in the U.S. House of Representative's single article of impeachment of inciting an insurrection at the "Save America" rally shortly before participants stormed the U.S. Capitol. It was the second time Trump has been impeached.
Giuliani appeared with Trump at the rally where he called for "trial by combat." It is unclear whether Giuliani himself will face criminal charges for his participation in the riot.
Giuliani has taken the lead as legal counsel on Trump's unsuccessful campaign to overturn the results of the 2020 election and deny the win to President-elect Joe Biden. More than 60 lawsuits filed by the president's team have been thrown out of court by local and federal judges and the U.S. Supreme Court.
The U.S. Senate will hold its second impeachment trial for Trump in upcoming days, likely soon after the inauguration of Biden. The trial could take several weeks or even months.
Trump has not picked his trial attorneys, White House adviser J. Hogan Gidley said in a tweet on Sunday.
"President Trump has not yet made a determination as to which lawyer or law firm will represent him for the disgraceful attack on our Constitution and democracy, known as the 'impeachment hoax,'" Gidley tweeted.
Trump plans to move to his Florida resort, Mar-a-Lago on Wednesday and would have to return to Washington, D.C., for the upcoming trial.