"Our democracy is what is at stake", Pelosi said.
"The facts are uncontested: The president abused his power for his own political benefit", she said. "The president abused his power for his own personal political benefit at the expense of our national security".
They allege the president withheld nearly $400 million (€360 million) in security assistance to pressure the government there into announcing an investigation involving former Vice President Joe Biden to benefit Trump's 2020 re-election campaign.
Speaking in somber tones against a backdrop of American flags, the California Democrat said Trump undermined U.S. security and jeopardized the integrity of the country's elections by pressuring a foreign government for help in next year's presidential campaign.
Ukraine wanted the aid, and a White House visit for President Volodymyr Zelenskiy, as a demonstration of U.S. support as it confronts Russian aggression. "Today, I'm asking our chairmen to proceed with articles of impeachment".
He said he wants to get on to a "fair trial" in the Senate.
Pelosi's announcement comes a day after four legal scholars testified to the House Judiciary Committee about whether Trump's actions related to the Ukraine scandal rise to the level of impeachment.
Pulling from the House's investigation, Democrats are focusing on at least three areas that could result in two to five articles, they say.
The chairmen of the House committees conducting the impeachment inquiry will begin drafting the articles, and some politicians are expecting to remain in Washington over the weekend.
Mr Trump has since fired back and accused the Democrats of trying to impeach him over "two totally appropriate (perfect) phone calls with the Ukrainian President".
Trump admitted that he had brought up Biden's family during the call but told reporters that he did so because "we don't want our people like Vice President Biden and his son creating to the corruption already in the Ukraine".
The White House has sent emissaries to Congress to bolster what had been a solid wall of support for Trump among Republicans.
Mr Trump has lambasted the impeachment probe daily and proclaims his innocence of any wrongdoing, but he has declined to testify before House hearings or answer questions in writing.
While liberal Democrats are pushing the party to incorporate the findings from former special counsel Robert Mueller's report on Russian interference in the 2016 election and other actions by Trump, more centrist and moderate Democrats prefer to stick with the Ukraine matter as a simpler narrative that Americans understand.