Abbe Pierre, 83, said he "forcefully" condemned anyone who tried to "deny, falsify or play down the Holocaust".
The priest, better known for his extreme left-wing views, issued his statement after meeting the Chief Rabbi of France and the President of the Board of Deputies of French Jews.
The controversy erupted when Abbe Pierre, who fought in the Resistance and helped Jews escape abroad, went into print in support of Garaudy's latest book, The founding myths of Israeli politics, which argues that the six million Jewish deaths in concentration camps is an exaggeration, used by Israel to foster a sense of nationhood. He claims many Jews died in epidemics and bombings rather than in death-camps.
The abbe, a friend of Garaudy, told Liberation there should be a Holocaust debate, so that Jewish deaths could be placed in the context of the "20 or 30 million people" the Nazis killed. He admitted, however, that he had only read a summary of the book.