WASHINGTON — President Donald Trump named Robert O'Brien as his national security advisor on Wednesday, eight days after the abrupt departure of John Bolton.

O'Brien, the State Department's top hostage affairs official, will become Trump's fourth national security advisor, following the departures of Bolton and retired Army Gens. Michael Flynn and H.R. McMaster.

Trump tweeted

While O'Brien's recent work primarily dealt with matters involving U.S. hostages, he was assigned by Trump to help free rapper A$AP Rocky, who was detained in Sweden on assault charges over the summer. The rapper was found guilty, but was spared from jail time.

O'Brien was also a partner in a boutique law firm focusing on high-stakes trials and appeals, and was a foreign policy advisor to Mitt Romney during the Utah senator's presidential run against then-President Barack Obama in 2012.

O'Brien attended an Oval Office event in March, where Trump greeted a freed U.S. citizen who had been held in Yemen. The president called O'Brien "the greatest hostage negotiator in our history" at that event.

Trump on Tuesday had named O'Brien and four other finalists for the role, including Vice President Mike Pence's national security advisor Keith Kellogg and Bolton's former chief of staff, Fred Fleitz.

In selecting O'Brien, Trump said: "I have worked long & hard with Robert. He will do a great job!"

Pence tweeted congratulations to O'Brien later Tuesday.

PENCE TWEET

The national security advisor does not require Senate confirmation. The position comes with a staff of hundreds of specialists from the Pentagon, State Department and U.S. intelligence agencies.

The White House, Pentagon and State Department did not respond to CNBC's requests for comment on the president's announcement.

Bolton, who was named national security advisor to succeed McMaster in March 2018, was known for his hardline approach to foreign policy, including calls for a pre-emptive strike on Iran and a hawkish position on North Korea. Trump announced on Sept. 10 that he fired Bolton saying that his advisor "disagreed strongly with many of his suggestions." Bolton said he resigned.