Trump made the revelation in Cincinnati on Thursday evening during his first post-election rally, hours after The Washington Post first reported the selection.

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“We are going to appoint ‘Mad Dog’ Mattis as our secretary of Defense,” Trump told the crowd. “But we’re not announcing until Monday, so don’t tell anybody.”

Trump had publicly praised Mattis after their meeting at his golf course in New Jersey last month, calling the former military official "the real deal."

Mattis retired in spring 2013 after leading U.S. Central Command, overseeing the U.S. military efforts in the Middle East. He has more recently worked as a national security fellow at the California-based Hoover Institution.

Mattis will need special permission from Congress to serve as secretary of Defense: a waiver of the seven-year buffer required of former military officers before they lead the Pentagon.

Trump's transition team held off on a formal announcement of Mattis's selection before the rally Thursday night. The president-elect's announcement came less than four hours after transition spokesman Jason Miller denied the Post's report that Trump had settled on Mattis.

“No decision has been made yet with regard to Secretary of Defense,” Miller wrote on Twitter.

Trump met with Mattis nearly two weeks ago at the president-elect’s golf course in Bedminster, N.J., while vetting several potential Cabinet picks. Afterward, Trump offered praise of the retired general, leading many to consider him the leading candidate for the Defense position. Trump also sang Mattis’s praises in an interview with The New York Times last week, saying he was swayed by the Marine’s argument against torturing terror suspects. “He said, ‘I’ve always found, give me a pack of cigarettes and a couple of beers and I do better with that than I do with torture,'" Trump said of their meeting. "And I was very impressed by that answer. I was surprised, because he’s known as being like the toughest guy.”

Updated at 8:55 p.m.