O'Brien is currently serving as the State Department's special presidential envoy for hostage affairs.
President Donald Trump on Wednesday named Robert O'Brien, a State Department official who has specialized in hostage issues, as his new national security adviser.
"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," Trump tweeted. "I have worked long & hard with Robert. He will do a great job!"
O'Brien will replace John Bolton, whom Trump fired last week after a string of disagreements.