Justice Juan Merchan said the $US1000 ($A1500) fines he has imposed so far do not seem to be deterring Trump from violating the gag order which prohibits him from making public comments about jurors, witnesses and families of the judge and prosecutors if the statements mean to interfere with the case.
Merchan said jail time would be a last resort and one that he was trying to avoid at all costs.
But he said Trump's "continued, wilful" violations of the gag order amounted to a "direct attack on the rule of law".
"I do not want to impose a jail sanction and have done everything I can to avoid doing so. But I will if necessary," Merchan said from the bench in the absence of the jury.
Merchan said he considered jail time a "truly the last resort" for many reasons, including the disruption to the trial, political implications of jailing a leading presidential candidate ahead of an election and the extraordinary security challenges of incarcerating an ex-president with a lifetime Secret Service detail.
Merchan had previously fined Trump $US9000 for nine social media posts that he ruled had violated the gag order.
Merchan spoke as Trump sat at the defendant's table in the New York courtroom in the first criminal trial of a former US president.
Trump's criminal hush money trial, entering its 12th day, has featured testimony from a top aide and a former tabloid publisher about efforts during his first presidential bid to tamp down stories of unflattering sexual behaviour.
Donald Trump complains that his trial has prevented him from campaigning in his White House bid. (AP PHOTO)
New York prosecutors have charged Trump with falsifying business records to cover up a $US130,000 payment to porn star Stormy Daniels, who claims to have had a sexual encounter with him in 2006.
Trump has pleaded not guilty and denies ever having sex with Daniels.
Trump complains frequently that the first criminal trial of a former US president has kept him cooped up in a chilly Manhattan courtroom when he should be out wooing voters as he mounts a comeback White House bid.
