"We are going to appoint 'Mad Dog' Mattis as our Secretary of Defence," Trump told a cheering crowd at Cincinnati in Ohio in his first of the series of 'Thank You' victory tour.
"But we are not announcing this till Monday, so don't tell anybody," Trump told his supporters in his characteristic way.
Trump described 66-year-old Mattis as "one of our great, great generals."
"He's our best. They say he's the closest thing to (World War II-era) general George Patton that we have and it's about time," Trump said at the rally.
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Tough-talking Mattis attracted controversy in 2005 when he said "it's fun to shoot some people" while addressing service members in San Diego.
If confirmed by the US Senate, Mattis would replace outgoing Defence Secretary Ashton Carter.
The nomination comes after Mattis met with Trump in Bedminster, New Jersey, last month.
Trump had said he changed his position on enhanced interrogation after meeting Mattis.
"He said -- I was surprised -- he said, 'I've never found it to be useful.' 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.