President Trump on Wednesday named the State Department's hostage envoy Robert C. O’Brien as his fourth national security adviser, replacing John Bolton, who was ousted from the administration last week.

“I am pleased to announce that I will name Robert C. O’Brien, currently serving as the very successful Special Presidential Envoy for Hostage Affairs at the State Department, as our new National Security Advisor. I have worked long & hard with Robert. He will do a great job!” Trump tweeted Wednesday morning.

TRUMP OUSTS NATIONAL SECURITY ADVISER JOHN BOLTON, SAYS THEY 'DISAGREED STRONGLY' ON POLICY

Trump's appointment is not subject to Senate confirmation.

O’Brien joined the administration in May 2018 and serves as the Special Presidential Envoy for Hostage Affairs at the State Department. O’Brien, who served under Secretary of State Mike Pompeo, led the U.S. government’s diplomatic efforts on overseas hostage-related matters, working closely with families of American hostages, as well as working with the administration on developing and implementing hostage recovery policy and strategy.

Most recently, O’Brien was involved in the high-profile arrest of American rapper A$AP Rocky. Over the summer, O’Brien traveled to Sweden to monitor court proceedings after the rapper pleaded not guilty to assault after a street fight that landed him in jail in Stockholm. He was released in August.

O’Brien also served during the Obama administration as co-chairman of the State Department’s public-private partnership for Justice Reform in Afghanistan.

O'Brien will be the president's fourth national security adviser.

O’Brien’s appointment comes just days after Trump announced that he had fired Bolton—whom O’Brien worked with in 2005 when he served as U.S. Representative at the United Nations during former President George W. Bush’s administration.

“I informed John Bolton last night that his services are no longer needed at the White House,” Trump tweeted last Tuesday. “I disagreed strongly with many of his suggestions, as did others in the Administration and therefore I asked John for his resignation, which was given to me this morning. I thank John very much for his service. I will be naming a new National Security Adviser next week.”

FEVERED SPECULATION OVER JOHN BOLTON'S REPLACEMENT AS NATIONAL SECURITY ADVISER

Bolton swiftly challenged Trump’s version of events, saying he offered to resign. The two had well-known disagreements on a range of hot-button national security issues, perhaps most significantly on plans for a troop drawdown in Afghanistan.

While Trump announced a 4,000-troop increase in 2017 as part of an effort to break the stalemate in the country, he has been moving toward agreeing to a phased withdrawal of troops. Some 14,000 U.S. troops have remained in Afghanistan, advising and assisting Afghan forces and conducting counterterrorism operations.

Inside the administration, Bolton also advocated caution on Trump's strategy with North Korea and was against Trump's decision last year to pull U.S. troops out of Syria. Bolton also led a quiet effort inside the administration and with allies abroad to convince the president to keep U.S. forces in Syria to counter ISIS and Iranian influence in the region.

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Bolton later told Fox News that his "priority has always been policies that make America secure. Nothing more, nothing less."

Bolton, who previously served as a Fox News contributor, worked in the administrations of former Presidents Ronald Reagan, George H.W. Bush and George W. Bush and served as George W. Bush's lawyer during the 2000 Florida recount. Bolton also served as a U.S. permanent representative to the United Nations from 2005 to 2006, and as an undersecretary of state for arms control and international security from 2001 to 2005.

Bolton replaced former National Security Adviser H.R. McMaster in 2018. McMaster replaced former National Security Adviser Gen. Michael Flynn.

O'Brien's selection was praised by the family of Robert Levinson, who was allegedly taken hostage by Iranian authorities while visiting Kish Island, Iran in March 2007.

"Our family is very glad to hear of Robert O'Brien's appointment," the statement said. "He has been a strong advocate within the U.S. government for our father, Robert Levinson. This is further evidence of President Trump's commitment to bringing home Americans held abroad. We look forward to continuing to work with Ambassador O'Brien and the NSC to bring Bob Levinson home to those who love him."

The Associated Press contributed to this report.