WASHINGTON—President Trump knew about Democratic campaign emails stolen by Russian hackers before they were made public during the 2016 presidential race, Omarosa Manigault Newman, a former top White House aide who was fired in December, claimed Tuesday.

Ms. Manigault Newman, who is promoting a new book, also said she has been interviewed by special counsel Robert Mueller, who is investigating whether the Trump campaign and Russian operatives colluded in helping defeat Hillary Clinton in the election. The campaign and Moscow deny those allegations.

Speaking on MSNBC, Ms. Manigault Newman was asked if Mr. Trump knew about the emails before WikiLeaks began releasing them in the summer of 2016. “Absolutely,” she said.

Ms. Manigault Newman’s assertion contradicts the president’s statements that he had no advance knowledge of the messages that created a drag on the Clinton candidacy. She didn’t provide evidence to support her contention and there was none in her book, “Unhinged,” which was released Tuesday.

Rudy Giuliani, one of Mr. Trump’s attorneys, denied that Mr. Trump knew in advance the emails were coming out. “She’s definitely lying to sell her books,” Mr. Giuliani said.