His announcement came a day after President Trump said he was withdrawing troops from Syria – a decision general Mattis is understood to oppose.
In his resignation letter, General Mattis strongly hinted at policy differences with Mr. Trump.
He said the president had the right to appoint someone “whose views are better aligned with his.
68-year-old General Mattis, will leave the job in February.
President Trump has not immediately named a successor, but said one would be appointed shortly.