The endorsement came as Sen. Hillary Rodham Clinton widened her lead over fellow Democratic presidential candidates and over Giuliani in New York. The latest poll has her leading Giuliani, the former New York City mayor, 52 percent to 39 percent. She had just a 5-point lead in April.
President Clinton appointed Freeh, a former federal judge, to become the nation's top lawman in 1993. But the relationship between the two men soured over the White House scandal investigations and what Freeh later claimed was Clinton's soft stance on terrorism.
The Freeh endorsement is viewed by supporters as a boost to Giuliani's image as a strong leader against terrorism and crime in the wake of the 9/11 terrorist attacks. On the campaign trail, Giuliani has asserted that he and his fellow Republicans have the best approach to deterring terrorism.
"Rudy Giulani's optimistic leadership is responsible for making the city of New York what it is today, one of the safest largest cities in the country and a place where the world feels safe to visit," Freeh said in a statement prior to a news conference.
"No one knows better than Louie Freeh what it takes to fight crime," said Giuliani.
Freeh, who lives in Wilmington, Del., also will serve as senior homeland security adviser for Giuliani's campaign and will head the candidate's Delaware campaign.
The two have known each other since the 1970s; Freeh prosecuted New York mafia cases under Giuliani, who was then the U.S. attorney in Manhattan.
Freeh has long pushed to expand U.S. intelligence gathering overseas and was responsible for positioning FBI agents as liaison in key countries around the globe.
He served eight years as FBI director before resigning in June 2001, and is now president of Freeh Group International, a legal affairs consulting firm.