Deputy Defence Secretary Patrick Shanahan has been appointed acting Pentagon chief, President Donald Trump has announced.
The move follows the resignation of James Mattis, who hinted at policy differences with Trump when he stepped down on Friday.
General Mattis, 68, said the president had the right to appoint someone "whose views are better aligned with yours".
Shanahan will take over on 1 January, an earlier date than expected.
Gen Mattis had been expected to leave the job in February, but on Sunday it was reported that the president had been considering removing him early.
"Patrick has a long list of accomplishments," Trump tweeted when announcing his decision. "He will be great!"
Shortly after posting this, he shared details of a "productive call" he had had with President Recep Tayyip Erdogan of Turkey.
Trump said the pair had discussed "our mutual involvement in Syria, & the slow and highly coordinated" withdrawal of US troops from the country.
Gen Mattis resigned shortly after President Trump announced the decision to withdraw all troops from Syria.
While not mentioning it in his resignation letter directly, he is understood to oppose the decision and has previously warned that an early withdrawal would be a "strategic blunder