Robert O’Brien has had a long career in foreign policy working for both main US parties and currently heads hostage negotiations at the State Department.
He is Mr Trump’s fourth national security adviser of his presidency.
As top adviser on national security, Mr O’Brien will be tasked with tackling everything from immigration policy to heightened tensions with Iran.
“I have worked long & hard with Robert. He will do a great job!” Mr Trump said in his tweet announcing Mr O’Brien’s selection.
He follows Mr Bolton, HR McMaster and Michael Flynn in the role, which does not require confirmation by Congress.
So, who is Robert O’Brien?
The new national security adviser trained as a lawyer then went on to advise Republicans on foreign policy and has served in various government roles.
Former President Bush appointed him in 2005 to serve as a representative to the UN general assembly.
While at the UN, he worked with Mr Bolton, who was then the ambassador. He has also served worked for former secretaries of state Condoleezza Rice and Hillary Clinton.
In his role as special presidential envoy for hostage affairs, Mr O’Brien works with families of US hostages and advises on related issues, including recovery policies.
US media report that Mr O’Brien impressed Trump with his hostage negotiation skills.
Mr O’Brien was involved with the release of US rapper ASAP Rocky from Swedish detention last month.