In a written order, Justice Juan Merchan said a $US9000 ($A13,9000) fine may not be enough to serve as a deterrent for the wealthy businessman-turned-politician and lamented he did not have the authority to impose a higher penalty.
"Defendant is hereby warned that the Court will not tolerate willful violations of its lawful orders and that if necessary and appropriate under the circumstances, it will impose an incarceratory punishment," Merchan wrote.
Merchan had imposed the gag order to prevent Trump from criticising witnesses and others involved in the case.
Donald Trump is charged with falsifying records to conceal $US130,000 paid to Stormy Daniels. (AP PHOTO)
The fine - $US1000 for each of nine online statements that Merchan said violated the order - was just short of the $US10,000 penalty that prosecutors had requested for posts that insulted likely witnesses and questioned the impartiality of the jury.
Merchan will consider whether to impose further fines for other statements at a hearing on Thursday.
The judge also ordered Trump to remove the statements from his Truth Social account and his campaign website by 2.15pm on Tuesday.
Merchan's order came as the trial in New York resumed with testimony from a banker familiar with accounts involved in Trump's alleged scheme to influence the 2016 election by covering up a sex scandal.
The Republican candidate in the 2024 presidential election is charged with falsifying business records to conceal a $US130,000 payment to porn star Stormy Daniels in exchange for her silence about a sexual encounter she said she had with Trump in 2006.
Trump has pleaded not guilty and denied having sex with Daniels, whose real name is Stephanie Clifford.
The historic criminal trial is the first of a former US president and began on April 22.
Roughly two dozen Trump supporters rallied outside the courthouse on Tuesday morning, chanting his name and waving banners that read "TRUMP 24".
A local Republican organisation had called for supporters to turn out after Trump complained that few people were protesting the trial.
Banker Garry Farro, who is not accused of wrongdoing, testified on Friday about financial records filed by Trump's one-time lawyer and fixer Michael Cohen, who prosecutors say helped carry out the scheme.
Trump is required to attend the trial and has said he could instead be campaigning ahead of his rematch with Democratic President Joe Biden in the November 5 election.
The criminal case is one of four pending against Trump but could be the only one to go to trial and result in a verdict before the election.
