The religious broadcaster announced he backs Giuliani, who supports abortion and gay rights, during an appearance at the National Press Club in Washington, the Washington Post reported.
The move is seen as a blow to GOP conservative candidate Mitt Romney, who has been working hard to garner the Christian Coalition founder's backing.
Robertson's support, sought by several presidential candidates, gives Giuliani a major lift among social conservatives as the former New York City mayor tries to convince Republicans he is an acceptable choice as the party's presidential nominee.
Besides founding the coalition, Robertson also founded the Christian Broadcasting Network and Regent University in Virginia Beach, Va. He ran for president in 1988, surprising observers by finishing second in the Iowa caucuses before his campaign began to lose steam.