The President-elect described the former marine as 'our best;
President-elect Donald Trump has named General James 'Mad Dog' Mattis as his Defence Secretary.
General Mattis is only the second retired general to become the civilian leader of the military.
The former marine, who served in Iraq and Afghanistan, has been described by the US President-elect as "our best".
“We are going to appoint 'Mad Dog' Mattis as our secretary of defense … but we are not announcing it until Monday, so don’t tell anyone. He’s great," Mr Trump said at a rally in Cincinnati last night.
General Mattis has publicly criticised Barack Obama's administration's policy on the Middle East, particularly over Iran, which he has called “the single most enduring threat to stability and peace in the Middle East".
He earned the nickname 'Mad Dog' because of his use of salty language and his dedication to the military. He previously co-authored a counterinsurgency strategy manual credited with helping to halt some of the worst sectarian violence in Iraq before the U.S. military withdrawal in December 2011.
However, similar to Vice President-elect Mike Pence, he struggles with Trump's stance on Russia, having previously saying he thinks Russia wants to "break NATO apart".
He also criticised Mr Trump throughout his presidential campaign, particularly when he said that NATO was obsolete and that its members don't pay their fair share.
On immigration, he said Mr Trump's desire for an all-out ban on Muslims immigrants is "causing us great damage right now, and it’s sending shock waves through this international system".
He's also in disagreement with a lot of his fellow generals on military issues. At the Marines' Memorial Club in San Francisco, he expressed reservations about allowing women into the military, saying that "the idea of putting women in there is not setting them up for success".
General Mattis was a firm favourite of former Republican nominee Senator John McCain for the position.
"General Mattis is one of the finest military officers of his generation and an extraordinary leader who inspires a rare and special admiration of his troops," McCain said in a recent statement. "He is a forthright strategic thinker. His integrity is unshakable and unquestionable. And he has earned his knowledge and experience the old-fashioned way: in the crucible of our nation's defense and the service of heroes.
"General Mattis has a clear understanding of the many challenges facing the Department of Defense, the U.S. military and our national security," McCain added.