President Donald Trump has said he is making Deputy Defence Secretary Patrick Shanahan acting Pentagon chief.
The President said he will be replacing Jim Mattis on January 1, after he quit following the proposed withdrawal of US troops from Syria.
President Trump, writing on Twitter, said: "I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019.
"Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!"
Lt. Col. Joseph Buccino, a spokesman for Mr Shanahan, said the former Boeing executive will accept the appointment as acting secretary.
"Deputy Secretary will continue to serve as directed by the president, and the Department of Defense will remain focused on the defense of the nation," he said on Sunday.
Mr Mattis previously resigned over policy differences, though he offered to stay in his post for two more months.
His resignation letter implicitly criticised President Trump's foreign policies, as well as his treatment of military allies after his decision to pull troops out of Syria.
On Saturday a US State Department official confirmed Brett McGurk, special envoy for the global coalition to defeat ISIS, had also handed in his resignation.
The 45-year-old also referenced the Syria move as being behind his decision.
Criticism has been levied at President Trump from a number of sources, following his shock Syrian announcement on Wednesday.
This was made via a video on Twitter and the President is acting to pull all 2,000 US troops from Syria.
He also declared victory over the Islamic State, contradicting his own experts' assessments.
He has stood by his decision, saying he should be being praised for it.
French President Macron said the situation was regrettable and that "an ally should be dependable".
