President Donald Trump is replacing national security adviser HR McMaster with John Bolton, a former US ambassador to the United Nations.
Mr Trump tweeted that Mr McMaster has done “an outstanding job & will always remain my friend.” He said Mr Bolton will take over on April 9.
The president has repeatedly clashed with Mr McMaster, a respected three-star general, and talk that he would soon leave the administration had picked up in recent weeks.
His departure follows Mr Trump’s dramatic ousting of Secretary of State Rex Tillerson last week.
In a statement released by the White House, Mr McMaster said he would be requesting retirement from the US Army effective from this summer, adding that afterwards he “will leave public service”.
The White House said Mr McMaster’s exit had been under discussion for some time and stressed it was not due to any one incident.
Mr Bolton, probably the most divisive foreign policy expert ever to serve as UN ambassador, has been a hawkish voice in Republican foreign policy circles for decades.