The House speaker laid out details of the move today and said: "Our democracy is what is at stake.
"The president leaves us no choice but to act."
A house committee has been conducting an impeachment inquiry into Mr Trump, which has laid the foundations for the latest action.
The process could lead to Mr Trump being ousted from his position.
Though such a step would eventually need the backing of the Republican controlled Senate, after passing the Democratic-held House of Representatives, and it would appear unlikely Mr Trump's own party would do so.
Mr Trump has long branded the inquiry a "witch hunt" and has pleaded his innocence throughout.
Today's action comes the day after Mr Trump returned from a Nato summit in London.
Earlier today, he hit out at the Democrats and said they have "no impeachment case and are demeaning our country".
Writing on twitter he added: "But nothing matters to them, they have gone crazy. Therefore I say, if you are going to impeach me, do it now, fast, so we can have a fair trial in the Senate, and so that our Country can get back to business."
Ms Pelosi delivered the historic announcement as Democrats push towards a vote, which could possibly be held by Christmas.
At the centre of the impeachment probe is a call with the president of Ukraine which occurred in July.
In this he asked the Ukraine to investigate rival Democrats including Joe Biden and spoke of withholding aid from the ally, which faced an aggressive Russia on its border.