Former UN Ambassador John Bolton will serve as the new national security adviser, replacing HR McMaster.
US President Donald Trump made the announcement on Twitter on Friday morning (Thursday local time) that he was ousting Mr McMaster for Mr Bolton, who will take over on April 9.
Mr Bolton, who has advocated using military force against Iran and North Korea, and has taken a hard line against Russia, will be Mr Trump’s third national security adviser in 14 months.
I am pleased to announce that, effective 4/9/18, @AmbJohnBolton will be my new National Security Advisor. I am very thankful for the service of General H.R. McMaster who has done an outstanding job & will always remain my friend. There will be an official contact handover on 4/9.
— Donald J. Trump (@realDonaldTrump) March 22, 2018
Mr Bolton is touted as a polarising figure within Washington foreign policy circles, having served the Republican administrations of Ronald Reagan, George HW Bush and George W Bush.
Under President W Bush, Mr Bolton was a leading advocate of the 2003 invasion of Iraq as the State Department’s top arms control official.
The invasion was later found to have been based on bogus and exaggerated intelligence about president Saddam Hussein’s weapons arsenal and ties to terrorism.
In a statement released by the White House, Mr McMaster said he would be requesting retirement from the US Army effective this summer, adding that afterward 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.
President Trump tweeted that Mr McMaster has done “an outstanding job & will always remain my friend”.
His departure follows the President’s dramatic ousting of Secretary of State Rex Tillerson last week, and the resignation of his lead lawyer for special counsel Robert Mueller’s investigation, John Dowd, on Friday.
Tension between Mr Trump and Mr McMaster has grown increasingly public in recent months.
In February, Mr Trump was angered by Mr McMaster’s characterisation of Russian meddling in the 2016 election after the national security adviser told the Munich Security Summit that interference was beyond dispute.
“General McMaster forgot to say that the results of the 2016 election were not impacted or changed by the Russians and that the only Collusion was between Russia and Crooked H, the DNC and the Dems,” Mr Trump tweeted February 17, alluding to frequent GOP allegations of impropriety by Democrats and Hillary Clinton.
Mr McMaster told The New York Times last year that Mr Trump’s unorthodox approach “has moved a lot of us out of our comfort zone, me included”.