"I disagreed strongly with many of his suggestions, as did others in the Administration", said Trump, without getting into specifics.
The timing of Trump's announcement came as a surprise, given that an hour earlier the White House had announced a Tuesday press briefing on an unspecified briefing, which was to feature Bolton and Secretary of State Mike Pompeo.
Trump's take was that he told Bolton last night that he was no longer needed, and Bolton then gave his resignation this morning.
The firing comes after months of tension over foreign policy between the pair, with Mr Trump at times publicly mocking Mr Bolton for his hawkish approach.More news: Google Assistant On The Pixel 4 Will Hold Calls For You
More news: Robert Mugabe set to receive state funeral at Zimbabwe sports stadium
More news: USA to preserve economic expansion
I thank John very much for his service. He was Trump's third national security advisor since his election in late 2016.
The president and Bolton reportedly had clashed on North Korea and Afghanistan in recent weeks.
Inside the administration he advocated caution on the President's whirlwind rapprochement with North Korea and against Mr. Trump's decision previous year to pull US troops out of Syria.
Bolton was Trump's third national security adviser, and his removal comes hot on the heels of a freaky announcement by Trump at the weekend.