The second impeachment trial of Donald Trump, the 45th president of the United States (in office from 2017 to 2021), began on February 9, 2021, and concluded with his acquittal on February 13. Donald Trump had been impeached for the second time by the House of Representatives on January 13, 2021. The House adopted one article of impeachment against Trump: incitement of insurrection. He is the only U.S. president and only federal official to be impeached twice. He was impeached by the House seven days prior to the expiration of his term and the inauguration of Joe Biden. Because he left office before the trial, this was the first impeachment trial of a former president. The article of impeachment addressed Trump's attempts to overturn the 2020 presidential election results (including his claims of election fraud and his efforts to pressure election officials in Georgia) and stated that Trump incited the attack on the Capitol in Washington, D.C., while Congress was convened to count the electoral votes and certify the victory of Joe Biden and Kamala Harris.

At the beginning of the trial, Senator Rand Paul forced a vote to dismiss the impeachment charge on the basis that it was unconstitutional to try a former president, arguing that impeachment only applies to current federal officers and that the punishment of removal from office was moot under the circumstances. Supporters of proceeding with the trial argued that the Constitution also permits disqualification from holding future office, which the House had requested in its article of impeachment. The motion was defeated in a 55–45 vote, with all Democrats, both independents, and five Republicans (Susan Collins of Maine, Lisa Murkowski of Alaska, Mitt Romney of Utah, Ben Sasse of Nebraska, and Pat Toomey of Pennsylvania) voting against the motion. This was the first time that a former president had been tried, and only the second time the Senate tried someone who had already left office, after Secretary of War William W. Belknap in 1876. Jamie Raskin was the lead impeachment manager and the primary author – along with Representative David Cicilline and Representative Ted Lieu – of the impeachment article, which charged Trump with inciting an insurrection by sparking the Capitol attack. Joaquin Castro, Eric Swalwell, Madeleine Dean, and Stacey Plaskett also assisted in delivering the oral arguments for conviction.

Trump's defense was led by Michael van der Veen, a personal injury lawyer from Philadelphia, along with David Schoen and Bruce Castor. Van der Veen's style and substance during the trial drew ridicule and criticism from many, with gasps and laughter in the Senate when he stated that he would seek to depose at least 100 people at his Philadelphia office, including Speaker of the House of Representatives Nancy Pelosi and Vice President Kamala Harris. Trump had originally hired Butch Bowers and Deborah Barbier to represent him, but they quit along with three other lawyers after "the former president wanted the lawyers representing him to focus on his allegations of mass election fraud" and his false claim that "the election was stolen from him."

At the conclusion of the trial, the Senate voted 57–43 to convict Trump of inciting insurrection, falling 10 votes short of the two-thirds majority required by the Constitution, and Trump was therefore acquitted. Seven Republican senators joined all Democratic and independent senators in voting to convict Trump, the largest bipartisan vote for an impeachment conviction of a U.S. president or former U.S. president. After the vote on the acquittal, Mitch McConnell said, "There's no question that President Trump is practically and morally responsible for provoking the events of the day." but he voted against conviction due to his interpretation of the United States Constitution.

Under the U.S. Constitution, the House has the sole power of impeachment (Article I, Section 2, Clause 5), and after that action has been taken, the Senate has "the sole Power to try all Impeachments" (Article I, Section 3, Clause 6). Trump was the third U.S. president to face a Senate impeachment trial, after Andrew Johnson and Bill Clinton. Trump is the only federal official to be impeached twice.

The Senate impeachment trial procedures are set forth under rules adopted in 1986, although specific rules are adopted for each trial, with the Senate majority leader exercising considerable power in setting the procedures.

On January 11, 2021, Representatives David Cicilline, Jamie Raskin, and Ted Lieu introduced an article of impeachment against Trump, charging him with "incitement of insurrection" in urging his supporters to march on the Capitol building. The article stated that Trump had committed high crimes and misdemeanors by making several statements that "encouraged – and foreseeably resulted in – lawless action" that interfered with Congress' constitutional duty to certify the election. It said he had "threatened the integrity of the democratic system, interfered with the peaceful transition of power, and imperiled a coequal branch of Government" in a way that rendered him "a threat to national security, democracy, and the Constitution". A total of 218 of the 222 House Democrats co-sponsored the article of impeachment, assuring its passage.

The House passed the article of impeachment on January 13, 2021, by a 232–197 vote. All 222 Democrats voted to impeach, joined by 10 Republicans (including House Republican Conference chairwoman Liz Cheney). Four Republicans did not vote, and the other 197 Republicans voted no.