Al Green, of Texas, told fellow representatives in Congress: "This is where I stand. I will not be moved. The President must be impeached," he said.
"The American people do not participate in democracy just on election day. I'm speaking to the American people. It's time to tell us where you stand. It's time to let us know."
- Read more
It signals mounting pressure on the President, following revelations he had pressured James Comey, then the director of the FBI, to drop the investigation into General Mike Flynn.
Mr Flynn, formerly the national security adviser, resigned over undisclosed contact with a Russian diplomat.
Detail of Mr Trump's intervention came in a memo, written by Mr Comey and read by the New York Times, in which he said the President had also suggested he try to imprison journalists who published classified information.
During a news conference on Monday afternoon, Mr Green accused the President of obstructing the ongoing investigation into his campaign’s ties to Russia and said "the mantra should be ITN: impeach Trump now".
The congressman initially said he would wait a few weeks before starting the impeachment process.
"It’s not the political expedient thing to do, this is what we must do to maintain our democracy," Mr Green said.
Earlier on Wednesday a former adviser to three presidents said Mr Trump was in "impeachment territory".
David Gergen, who advised presidents Clinton, Reagan and Nixon, told CNN: "After watching the Clinton impeachment, I thought I would never see another one. But I think we’re in impeachment territory for the first time. I think that the obstruction of justice was the number one charge against Nixon that brought him down.
"I'm a lapsed lawyer, I can not tell you if it meets all of the legal definitions, but I can tell you from a lay point of view, it looks like [Mr Trump] was trying to impede the investigation, he was using his power to do that, and when James Comey didn't go along with him, he wasn't his boy, he fired him, which I think is also relevant to the question of what he was trying to do.
"So, from my point of view, this is of enormous consequence for his presidency."
Meanwhile, David Axelrod, Barack Obama's campaign manager and former adviser, tweeted: "I've been resistant to impeachment talk until now, but if Comey memo is true - and Comey is very credible - we are into a whole new deal here."
Representative Jason Chaffetz, Republican-Utah, says in a letter to the FBI that he's making the request to determine whether the president attempted to influence or impede the bureau's investigation into Mr Flynn.
Mr Chaffetz's letter came just hours after the disclosure of Mr Comey's memo. His letter to acting FBI director Andrew McCabe requests all memoranda, notes, summaries and recordings that relate to any communications between Mr Comey and the President.
The letter gave the FBI a week to produce the records.