President Trump is replacing US National Security Adviser HR McMaster with Bush-era defence hawk John Bolton.
In a tweet, Mr Trump said he was thankful for Mr McMaster’s service and said he had done an „outstanding job“.
He is the latest in a series of high-profile departures from the White House.
Last week, Mr Trump fired Secretary of State Rex Tillerson via Twitter, replacing him with former CIA Director Mike Pompeo.
Mr Bolton, 69, has advocated military force against Iran and North Korea.
He will be Mr Trump’s third national security adviser in the 14 months of the administration and is to take over next month.