"I’ve just wanted to announce – we’ve been working all weekend, very diligent and very hard – that Gen. H.R. McMaster will become the national security adviser," Trump told reporters in Florida.
"As a man of tremendous talent and tremendous experience … he is highly respected by everybody in the military and we’re very honored to have him," the president said.
Lt. Gen. McMaster will take over as the director of the National Security Council, while Lt. Gen. Keith Kellogg will remain the council's chief of staff.
Announcing that Kellogg will hold on to his job, Trump said that, "I think that combination is something very, very special."
McMaster thanked the president, saying it was "a privilege to continue serving our nation."
"I am grateful to you for that opportunity and I’m looking forward to joining the National Security team and doing everything I can to advance and protect the interest of the American people," he said.
The appointment comes a week after Lt. Gen. Michael Flynn resigned over media reports that he inadvertently briefed Vice President Michael Pence and other White House officials with incomplete information about his phone conversations with the Russian ambassador to the United States before Donald Trump's inauguration.