It's the third national security advisers in 14 months.
U.S. President Donald Trump announced in a tweet he was ousting H.R. McMaster - who replaced Michael Flynn after a month - with John Bolton - an announcement Bolton says he was not expecting.
The long time polarizing figure in Washington foreign policy circles has advocated using military force against Iran and North Korea and has taken a hard line against Russia.
Trump's decision to replace McMaster was the latest high-level departure from the Trump administration.