Ailes stepped down from Fox News last month amid an investigation into sexual harassment allegations by several female employees. It is not clear when Ailes started aiding the campaign or if it is paying him, the Times said.
Trump spokeswoman Hope Hicks disputed the report to NBC News, saying it is "not accurate."
"He is not advising Mr. Trump or helping with debate prep. They are longtime friends, but he has no formal or informal role in the campaign," she said.
The debates, which start in late September, give Trump a crucial chance to prove his policy chops in a general election embodied more by his off-the-cuff comments than his solutions. Trump trails in most recent polls taken nationally and in battleground states ahead of November's election.
Ailes' involvement with the campaign could stretch beyond the debates, the Times reported. Ailes, who founded Fox News in 1996, previously was an adviser to Richard Nixon's 1968 presidential campaign and helped Ronald Reagan and George H.W. Bush prepare for debates in the 1980s.
Trump had a mixed relationship with Fox News during the GOP primary. He at times received apparently favorable coverage but had a public spat with Fox anchor Megyn Kelly.