In a letter to key lawmakers, Trump said he planned in 30 days to remove the official, Michael Atkinson, saying, "it is vital that I have the fullest confidence in the appointees serving as inspectors-general. This is no longer the case with regard to this inspector-general," he said.
A US official said Thomas Monheim, a career intelligence professional, would serve as acting inspector-general for the time being.
Atkinson, a Trump appointee, had determined that a whistleblower's report was credible in alleging that Trump abused his office in attempting to solicit Ukraine’s interference in the 2020 US election for his political benefit.
Atkinson was also concerned that Trump potentially exposed himself to “serious national security and counter-intelligence risks” when he pressed Ukrainian President Volodymyr Zelensky to investigate Democratic presidential hopeful Joe Biden and his son during a phone call last year on July 25, according to a Justice Department legal opinion.
After contentious partisan hearings, the Democratic-led House of Representatives voted to impeach Trump, but the Republican-led Senate acquitted him of the charges in early February.
Democrats heaped criticism on Trump for firing Atkinson, particularly as the country reels from the COVID-19 pandemic.
"In the midst of a national emergency, it is unconscionable that the president is once again attempting to undermine the integrity of the intelligence community by firing yet another intelligence official simply for doing his job," said Democratic Sen. Mark Warner, the ranking Democrat on the Senate Intelligence Committee.