Mattis served as a marine in Iraq and Afghanistan and has been vocal in airing his opposition to President Barack Obama's foreign policy on the Middle East, particularly Iran.
United States President-elect Donald Trump has nominated retired Marine Corps General James Mattis, known as "Mad Dog", for the key position of Secretary of Defense.
Trump's choice of Mattis to head up the Pentagon is being seen as an indication of his intention to radically change how the US deals with what it has seen as potential terror threats under the Obama administration's foreign policy.
Mattis, who had served as a marine in Iraq and Afghanistan, has been a staunch critic of Obama's foreign policy on the Middle East, particularly Iran.
Mattis has portrayed Iran as the "single most enduring threat to stability and peace in the Middle East."
"We are going to appoint "Mad Dog" Mattis as our Secretary of Defense," Trump told a cheering crowd in Cincinnati on Thursday, as he kicked off his "USA Thank You Tour 2016" for his supporters.
He said the formal announcement would be made on Monday.
While the nomination of 66-year-old Mattis will likely be popular among US forces, it will have to clear a bureaucratic hurdle.
Mattis would need the US Congress to waive a requirement that a defence secretary be a civilian for at least seven years before taking the top job at the Pentagon. He retired in 2013.
Trump described Mattis, as a "true general's general."
The New York real estate magnate famously asserted last year, "I know more about ISIS (Daesh) than the generals do." It is likely Mattis knows more.