After two days of deliberation, the 12-member jury pronounced Trump guilty on all 34 felony counts he faced.
Trump watched the jurors dispassionately as they were polled to confirm the unanimous verdict.
Justice Juan Merchan set sentencing for July 11, just days before the Republican Party is scheduled to formally nominate Trump for president ahead of the Nov. 5 election.
The crime of falsifying business documents carries a maximum sentence of four years in prison, though those convicted often receive shorter sentences, fines or probation. Incarceration would not legally prevent him from campaigning, or taking office if he were to win.
He will not be jailed ahead of sentencing.
The verdict plunges the United States into unexplored territory ahead of the November vote, when Trump will try to win back the White House from Democratic President Joe Biden.
Trump, 77, has denied wrongdoing and an attorney representing him said they would appeal as quickly as possible.
"This was a disgrace," Trump told reporters afterwards as he proclaimed his innocence and repeated his complaints that the trial had been rigged against him.
"The real verdict is going to be Nov. 5 by the people," he said.
Trump gave a thumbs-up sign through the tinted window of his SUV as his motorcade left the courthouse. Trump supporters stood in a park opposite the courthouse along with journalists, police and onlookers.
Opinion polls show Trump and Biden, 81, locked in a tight race, and Reuters/Ipsos polling has found that a guilty verdict could cost Trump some support among independent and Republican voters.
The case had been widely regarded as the least consequential of the four criminal prosecutions Trump faces. But the verdict looms large now as it is likely to be the only one before the election with the others delayed by procedural challenges.
The jury found Trump guilty of falsifying business documents after sitting through a five-week courtroom presentation that featured explicit testimony from porn star Stormy Daniels about a sexual encounter she says she had with Trump in 2006 while he was married to his current wife Melania. Trump denies ever having sex with Daniels.
Trump's former fixer Michael Cohen testified that Trump approved a $130,000 hush money payment to Daniels in the final weeks of the 2016 election, when Trump faced multiple accusations of sexual misbehavior.
Cohen testified that he handled the payment, and that Trump approved a plan to reimburse him through monthly payments disguised as legal work.
Trump's lawyers hammered Cohen's credibility, highlighting his criminal record and imprisonment and his history of lying. Merchan also cautioned jurors to examine his testimony carefully.
The relatively short amount of time jurors needed to reach a verdict was a sign that they thought there was enough evidence to back up Cohen's testimony, said George Grasso, a retired New York judge who attended the trial.
A source familiar with the Trump campaign's inner workings said the verdict was expected to prompt him to intensify deliberations on picking a woman as his vice presidential running mate. His campaign website labeled him a "political prisoner" and urged supporters to donate.
BIDEN CAMPAIGN: NO ONE ABOVE THE LAW
Biden's campaign said the verdict showed that no one was above the law and urged voters to reject Trump in the election.
"There is still only one way to keep Donald Trump out of the Oval Office: at the ballot box," the campaign said in a statement.
The White House declined to comment.
Trump's fellow Republicans quickly condemned the verdict. "Today is a shameful day in American history," House of Representatives Speaker Mike Johnson said in a prepared statement.
The jury notified the court they had reached a verdict at 4:20 p.m. (2020 GMT) and the foreperson read out all 34 guilty counts shortly after 5 p.m.
Trump lawyer Todd Blanche asked Merchan to throw out the guilty verdict, arguing that it was based on the unreliable testimony of Cohen. Merchan denied his request.
A Trump appeal is likely to focus on porn star Daniels' salacious testimony about their alleged sexual encounter as well as the novel legal theory prosecutors used in the case, but he faces long odds, legal experts said.
"We are going to appeal as quickly as we can. We will seek expedited review of this case," Trump attorney Will Scharf told Fox News.
As a standalone crime, falsifying business documents is normally a misdemeanor in New York, but prosecutors in Manhattan District Attorney Alvin Bragg's office elevated it to a felony on grounds that Trump was concealing an illegal campaign contribution.
They had the burden of proving Trump guilty "beyond a reasonable doubt," the standard under U.S. law.
"We did our job. (There are) many voices out there. The only voice that matters is the voice of the jury, and the jury has spoken," Bragg said.
Jurors heard testimony of sex and lies that have been public since 2018, although the charges themselves rested on ledger accounts and other records of Cohen's reimbursement.
It was known as the "zombie case" because Bragg brought it back to life after his predecessor opted not to bring charges.
If elected, Trump could shut down the two federal cases that accuse him of illegally trying to overturn his 2020 election loss and mishandling classified documents after leaving office in 2021. He would not have the power to stop a separate election-subversion case taking place in Georgia.
Trump has pleaded not guilty in all the cases, and has portrayed his various legal troubles as an effort by Biden's Democratic allies to hurt him politically.
