It comes after Lt Gen Michael Flynn resigned
President Donald Trump has named his new national security adviser and new security chief of staff.
Career military man Lieutenant General Herbert Raymond McMaster replaces Michael Flynn as national security adviser.
Mr Flynn resigned amid controversy over his dealings with Russia and accusations that he had lied to Vice President Mike Pence.
Meanwhile, Keith Kellogg will become chief of staff for national security.
Speaking at his Mar-a-Lago residence in Palm Beach, Florida, Mr Trump said the pair working together would be "very, very special".
The President called Lt Gen McMaster, who has specialised in counter-insurgency warfare, a "man of tremendous talent and tremendous experience".
Mr Trump added that he would ask John Bolton, former US ambassador to the United Nations, to work in his team in a "somewhat different capacity".
Lt Gen McMaster, whose appointment does not require Senate confirmation, said his new role was "a privilege".
During his long military career, the 54-year-old directed a transparency task force in Afghanistan and held key roles in the Iraq and Gulf wars.
He is also known for his vocal criticism of the US military's involvement in the Vietnam War and authored a book entitled "Dereliction of Duty: Lyndon Johnson, Robert
McNamara, the Joint Chiefs of Staff and the Lies That Led to Vietnam".
Mr Kellogg said he was "very honoured" by his appointment and paid tribute to Lt Gen McMaster, who he described as a "great statesman and a great soldier".
President Trump announced the appointments in a video on his personal Facebook page:
Last week, Mr Trump asked retired Vice Admiral Robert Harward to replace Michael Flynn as his national security adviser, but the former Navy Seal turned down the offer.
Mr Harward said it was "purely a personal issue", but US media suggested he wanted to bring in his own team.