According to CNN, Rivera says he is "filled with regret" for his instinctive support of Ailes, the Fox News chairman who resigned in July under a cloud of sexual harassment allegations.
"It became apparent that Gretchen was not alone in alleging abhorrent behavior behind his closed doors. Now I am filled with regret for stubbornly discounting their various allegations," Rivera wrote.
Rivera's message, published on Facebook on Thursday night, implied that he now believes Ailes did behave inappropriately.
"The Murdochs," the owners of 21st Century Fox, "would not have turned the world upside down but for good cause," he wrote while apologizing for his "skepticism."
"Like victims of sexual assault, those alleging harassment deserve the presumption of credibility," he wrote.
Rivera is one of more than a dozen prominent Fox reporters and hosts who rallied to Ailes' defense when Carlson sued in July.
Some of their comments came under severe scrutiny after Ailes resigned. Earlier this week, Fox settled Carlson's lawsuit to the tune of $20 million. The company issued an unusual public apology to her.
Fox has also completed private settlements with a "handful" of other women who accused Ailes of harassment.
Ailes, through his lawyers, has denied the allegations.