The justices, in a 6-3 ruling authored by Chief Justice John Roberts, threw out a lower court's decision that had rejected Trump's claim of immunity from federal criminal charges involving his efforts to undo his 2020 election loss to Joe Biden.
The ruling marked the first time since America's 18th century founding that the Supreme Court has declared that former presidents may be shielded from criminal charges in any instance.
Ex-president Donald Trump is accused of plotting an insurrection at the Capitol. (AP PHOTO)
"We conclude that under our constitutional structure of separated powers, the nature of presidential power requires that a former president have some immunity from criminal prosecution for official acts during his tenure in office," Roberts wrote.
Immunity for former US presidents is "absolute" with respect to their "core constitutional powers," Roberts wrote, and a former president has "at least a presumptive immunity" for "acts within the outer perimeter of his official responsibility," meaning prosecutors face a high legal bar to overcome that presumption.
In recognising broad immunity for Trump, Roberts cited the need for a president to "execute the duties of his office fearlessly and fairly" without the threat of prosecution.
"As for a president's unofficial acts," Roberts added, "there is no immunity."
Trump hailed the ruling in a social media post, writing: "BIG WIN FOR OUR CONSTITUTION AND DEMOCRACY. PROUD TO BE AN AMERICAN!"
Trump is the Republican candidate challenging Biden, a Democrat, in the US election in a 2020 rematch. The Supreme Court's slow handling of the case and its decision to return key questions about the scope of Trump's immunity to the trial judge to resolve make it improbable he will be tried before the November 5 election on the charges brought by Special Counsel Jack Smith.
Trump's trial had been scheduled to start on March 4 before the delays over the immunity issue. Now, no trial date is set. Trump made his immunity claim to the trial judge in October, meaning the issue has been litigated for about nine months.
In a separate case brought in New York state court, Trump was found guilty by a jury in Manhattan on May 30 on 34 counts of falsifying documents to cover up hush money paid to a porn star to avoid a sex scandal before the 2016 election. Trump also faces criminal charges in two other cases. He has pleaded not guilty in those and called all the cases against him politically motivated.
Justice Sonia Sotomayor wrote "the president is now a king above the law" after the ruling. (AP PHOTO)
Justice Sonia Sotomayor, joined by fellow liberal Justices Elena Kagan and Ketanji Brown Jackson, delivered a sharply worded dissent, saying the ruling effectively creates a "law-free zone around the president."
"When he uses his official powers in any way, under the majority's reasoning, he now will be insulated from criminal prosecution. Orders the Navy's Seal Team 6 to assassinate a political rival? Immune. Organises a military coup to hold onto power? Immune. Takes a bribe in exchange for a pardon? Immune. Immune, immune, immune," Sotomayor wrote.
"In every use of official power, the president is now a king above the law."
Biden criticised the Supreme Court ruling in forceful remarks at the White House.
"This nation was founded on the principle that there are no kings in America. Each, each of us is equal before the law. No one, no one is above the law. Not even the president of the United States," he told reporters.
With the Supreme Court decision, he said "that fundamentally changed."
US President Joe Biden sharply criticised the Supreme Court ruling. (AP PHOTO)
"Now the man who sent that mob to the US Capitol is facing potential criminal conviction for what happened that day. The American people deserve to have an answer in the courts before the upcoming election," Biden said, referring to Trump being on trial for his role in spurring the riot.
Biden said the public has a right to know the results of that prosecution before the election in November.
"Now, because of today's decision, that is highly, highly unlikely. It's a terrible disservice to the people in this nation."
Trump also faces election subversion charges in state court in Georgia and federal charges in Florida brought by Smith relating to keeping classified documents after leaving office.
If Trump regains the presidency, he could try to force an end to the prosecution or potentially pardon himself for any federal crimes.
