It was not clear whether the president was basing his tweet on direct knowledge, or on reports this week that special counsel Robert Mueller is examining whether he obstructed justice by firing Mr Comey last month amid the Russian Federation investigation.
Mr Trump was criticised for the tweet by the senior Democrat on the Senate Judiciary Committee, Dianne Feinstein. "I believe that the President will be better served - I know the country will be served by that".
Feinstein added, "That's undemocratic on its face and a blatant violation of the president's oath of office".
President Donald Trump continued a campaign on Twitter on Friday against the investigations into possible links between his campaign and Russian Federation.
Putin present to see Russia kick off Confederations Cup
The one-way traffic continued after the break, Marinovic denying Poloz and Aleksandr Erokhin with a fine double save. Bigger tests await - both in the short and long-term - but this was a very small step in the right direction.
What is clear is that he could order the Justice Department to axe Mr Mueller, which may result in Mr Rosenstein's departure and would certainly intensify the uproar over the investigation.
Hours later, a source close to Mr Trump's outside legal team said Mr Trump did not intend his tweet to be confirmation of the investigation but rather was reacting to a Washington Post story on Wednesday about the probe.
"As you and I have discussed, however, I can not defend the Director's handling of the conclusion of the investigation of Secretary Clinton's emails, and I do not understand his refusal to accept the almost universal judgment that he was mistaken".
At a congresional hearing last week, Comey confirmed that he told Trump three times that Trump was not being investigated as part of the Russia-collusion probe, and Trump tweeted "total and complete vindication" at that time.
A day earlier, Trump had questioned why his general election rival Hillary Clinton was not being investigated. "Regulations way down, jobs and enthusiasm way up!" said the US President in another tweet.
He has decided - after consulting with a small circle of advisers in multiple meetings over the last few days - that "this is a political fight and he's going to fight it", the official said.
The transition official spoke on condition of anonymity because the person was not authorized to discuss post-election decisions publicly. The White House denies any collusion.