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.

Who is Robert O'Brien?

Mr O'Brien has served as a major in the US Army Reserve. 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.

What happened to John Bolton?

Mr Bolton, a foreign policy hard-liner, was ousted after repeated clashes with the president and other members of the administration on foreign policy, particularly regarding North Korea and Venezuela.

Mr Trump tweeted on 10 September that he asked Mr Bolton for his resignation because he "disagreed strongly with many of his suggestions".

Moments after the president's announcement, Mr Bolton contested his version of events, saying that he had offered to resign but Mr Trump said they would "talk about it".

Sources said the National Security Council, which advises the president, had become a separate entity within the White House under Mr Bolton and many within the White House criticised Mr Bolton's approach.

Mr Bolton was the chief architect of the Trump administration's stance on Iran and also advocated for tougher policies on North Korea, Russia and Afghanistan.

After more than a year of having a political and ideological lightning rod as national security advisor, Donald Trump went with a much more conventional choice.

Where outgoing NSA John Bolton was bombastic - an outside personality with an eye for the television cameras - Robert O'Brien is a behind-the-scenes bureaucratic player. He spent much of his career practicing law, and when he did make forays into public service, it was mostly in roles deep in the machinery of government

Mr O'Brien, a civilian, also stands in contrast to the two former generals that the president has chosen for the position, Michael Flynn and HR McMaster. His selection could be an indication that Mr Trump is losing his early affinity for military brass.

Given that O'Brien was a foreign policy advisor for the Mitt Romney and Scott Walker presidential campaigns and worked in the George W Bush administration, Mr Trump's move could also help repair strained relations with the conservative foreign policy establishment.

Mr O'Brien takes this top national security position as multiple global disputes teeter on the brink of full-blown crisis. He will work for a commander-in-chief who prides himself on his unpredictability. The task ahead is daunting.