In early morning tweets, Mr Trump defended his bombshell decision to fire the FBI chief over his handling of the inquiry into Hillary Clinton’s emails.
Mr Comey was leading an FBI investigation into alleged links between the Trump campaign and Russia.
His removal has shocked Washington and Democrats are outraged.
The sacking “raises profound questions about whether the White House is brazenly interfering in a criminal matter,” said Adam Schiff, who is the highest ranked Democrat on the House Intelligence Committee.
But the president stood by his actions on Wednesday morning, hours before a meeting with Russian Foreign Minister Sergei Lavrov – his first with any Russian official since taking power.
“James Comey will be replaced by someone who will do a far better job, bringing back the spirit and prestige of the FBI,” said, Mr Trump.
“Comey lost the confidence of almost everyone in Washington, Republican and Democrat alike. When things calm down, they will be thanking me!” he added.
It is only the second time the head of the FBI has been fired.
During a surprise meeting with Henry Kissinger, Mr Trump gave his first in-person explanation of why he fired Mr Comey.
“Because he was not doing a good job,” he told reporters.