The decision came after a Republican state legislator wrote a letter to university President Alan G. Merten protesting the Fairfax school's plans to pay the filmmaker $35,000 to speak on Oct. 28.
"We just felt it wasn't the most appropriate use of (public) funds, so we decided the best thing to do was cancel," school spokesman Daniel Walsch said.
George Mason didn't notify Moore before making the decision public, Walsch said.
A message left seeking comment from Moore wasn't immediately returned to The Associated Press, but he told The Washington Post he plans to come and speak anyway.
"I'm going to show up in support of free speech and free expression," he said.
Loudoun County Del. Richard L. Black wrote a letter dated Tuesday urging Merten to reconsider the university's "lavish payment" to Moore, or to cancel the appearance.
"Tax money is being spent poorly, and for partisan purposes," wrote Black, who has one of the General Assembly's most conservative voting records.
Walsch said university officials didn't discuss with Moore whether they would allow him to speak if he waived his fee, nor did they approach student groups or other private organizations to come up with the money.
Moore's film, still playing in theaters, criticizes and ridicules President Bush's response to the Sept. 11, 2001, terrorist attacks and his decision to invade Iraq.