During an interview Tuesday with KCRA-TV, Comey suggested that impeaching Trump would be the wrong decision and that Democrats might stoke conspiracy theories by pursuing it.
"I think the American people would be let off the hook if Donald Trump were impeached and removed from office and a lot of his supporters would think some sort of coup had taken place," he said.
But on Wednesday, Comey tweeted that impeachment might be necessary for Congress to maintain its oath to uphold the Constitution.
"Some media are quoting me from last week about my hope that America will remove Donald Trump by an election, not Impeachment," he wrote.
"But we may now be at the point where members of the House and Senate can’t uphold their own oaths to support and defend the Constitution without acting."
Comey, a vociferous critic of Trump, has dismissed concerns about a "deep state" opposed to the president while challenging the idea that he and others plotted against then-candidate Trump in 2016.
"There was no corruption. There was no treason. There was no attempted coup. Those are lies, and dumb lies at that. There were just good people trying to figure out what was true, under unprecedented circumstances," he said in May.
While Comey didn't reveal the reason for his apparent about-face, his tweets came after the administration released a phone transcript that Democrats portrayed as a boost to their impeachment efforts.
The transcript, although incomplete, did not show an explicit quid pro quo on Trump's part but did show he and Ukrainian President Volodymyr Zelensky discussing corruption suspicions surrounding former Vice President Joe Biden and his son Hunter.
News of the conversation fueled Democratic calls for impeachment, with acting Director of National Intelligence Joseph Maguire scheduled to testify before the House Intelligence Committee Thursday.
Fox News' Joseph A. Wulfsohn contributed to this report.