Video report by ITV News Washington Correspondent Robert Moore
“Our democracy is what is at stake,” Ms Pelosi said. “The president leaves us no choice but to act.”
Ms Pelosi delivered the historic announcement as Democrats push toward a vote, possibly by Christmas.
She said she was authorising the drafting of articles of impeachment “sadly but with confidence and humility”.
“The president’s actions have seriously violated the Constitution,” Ms Pelosi said.
Watch Speaker Pelosi's speech in full
In her solemn announcement, Ms Pelosi added: “The president leaves us no choice but to act because he is trying to corrupt, once again, the election for his own benefit.
“The president has engaged in abuse of power, undermining our national security and jeopardising the integrity of our elections.”
At the heart of the impeachment probe is a July call with the president of Ukraine, in which Mr Trump pressed the leader to investigate Democrats as Mr Trump was withholding aid to the country.
In typical Trumpian style, the US President responded on Twitter.
He slammed the "Do Nothing, Radical Left Democrats" over the proceedings.
He redeclared his phone calls to Ukrainian officials were "perfect" and denounced Pelosi's quotation of the US founding fathers when speaking about their intentions for elements of the country's constitution.