Justice Juan Merchan set sentencing for July 11, days before the Republican Party is expected to nominate Trump for president ahead of the November 5 election.

Trump faces a maximum sentence of four years in prison, though others convicted of that crime often receive shorter sentences, fines or probation. Incarceration would not prevent him from campaigning, or taking office if he were to win.

He will not be jailed ahead of sentencing.

Merchan thanked the jurors for their service and told them they were free to speak about their experience on the jury if they wished. “Nobody can make you do anything you don’t want to do. The choice is yours,” Merchan said.

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 was expected to appeal.

"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 November 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 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 $US130,000 ($A196,000) hush money payment to Daniels in the final weeks of the 2016 election, when he faced multiple accusations of sexual misbehaviour.

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 labelled him a "political prisoner" and urged supporters to donate.

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.

The jury notified the court they had reached a verdict at 4.20pm on Thursday and read out all 34 guilty counts shortly after 5pm.

Falsifying business documents is normally a misdemeanour 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.

Trump had complained that he could not get a fair trial in his heavily Democratic hometown.

This case was also likely to be the only one to go to trial before the election, as the others are delayed by procedural challenges.

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.