In a vote on Wednesday, the Democratic-led House passed two charges against the 73-year-old, accusing him of an abuse of power and obstruction of Congress.
This followed an impeachment inquiry into allegations Mr Trump pressured Ukraine to investigate Democratic political rival Joe Biden and his son Hunter.
After Nancy Pelosi, the House of Representatives speaker, confirmed the votes, the US president condemned the impeachment as a “set-up” and a “hoax”, the New York Times reports.
The paper adds that, according to White House aides, “the day after found him still a little shellshocked”.
In public, Mr Trump has struck a defiant tone. Tweeting in capital letters, he said on Wednesday: “Such atrocious lies by the radical left, do nothing Democrats. This is an assault on America, and an assault on the Republican party!!!!”
Brad Parscale, the president’s campaign manager, called the Democrats who voted to impeach him “traitors” and urged supporters to fight back. “Let’s make sure they know that we will NEVER FORGET,” Parscale wrote.
Ivanka Trump, the president’s eldest daughter, made clear that her father was unfazed by Wednesday’s vote. “He’s energized,” she told CBS News.
Despite successfully impeaching Mr Trump, the Democrats do not have enough votes in the Republican-controlled Senate to convict and remove him from office.
Ms Pelosi is set to delay sending the articles to the Senate until Republican leaders offer more details about how they will handle an expected trial.
“We’re ready when we see what they have,” she told a news conference.
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