President Barack Obama accepted Defence Secretary Chuck Hagel's resignation on Monday, saying Mr. Hagel had approached him last month to say it was time for him to leave.
At a White House event in the State Dining Room, Mr. Obama and Vice-President Joe Biden gave Mr. Hagel a warm send-off.
Mr. Obama said Mr. Hagel will stay on until a successor is confirmed by the Senate.
He said Mr. Hagel has been candid with his advice and counsel and has "always given it to me straight."