US defence secretary Chuck Hagel resigns
President Barack Obama confirmed his resignation and paid tribute to his "class and integrity".
Mr Hagel, 68, a Vietnam war veteran and former Republican senator, said it had been his life's "greatest privilege".
Last month, it was reported that he was sharply critical of the US strategy against Islamic State and in relation to the Syria regime.
He will stay on as secretary until his successor is confirmed.
An unnamed official told the Associated Press news agency that Mr Hagel and Mr Obama both "determined that it was time for new leadership in the Pentagon" and had been discussing the move for weeks.
Mr Obama said Mr Hagel had come to him to discuss the "appropriate time to complete his service".
The US president said he was "extraordinary lucky" to have Mr Hagel by his side as defence secretary and praised him for forging a strong bond with US troops.
Mr Hagel is the first enlisted combat veteran to serve in the role.
In a brief statement, Mr Hagel said it had been the "great privilege of my life to lead and most importantly to serve the men and women of the defence department".