Paxton was seen walking with Trump before he entered the Manhattan criminal courthouse where Trump's trial is underway.
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Paxton said he would sit through the proceedings he called a sham.
He also posted to his social media account saying that the trial is a "travesty of justice" and that he "stands with Trump."
The attorney general did not make clear whether he is in New York for official state business or simply to attend the trial with Trump, with whom he has a close political relationship.
Trump was fined $9,000 for repeatedly violating a gag order that barred him from making public statements about witnesses, jurors, and some others connected to his New York hush-money case.
If he does it again, the judge warned, he could be jailed.
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Tuesday's ruling came at the start of the second week of testimony in the historic case, in which Manhattan prosecutors argue Trump and his associates took part in an illegal scheme to influence the 2016 presidential campaign by purchasing and then burying seamy stories.
The payouts went to a doorman with a torrid yarn; ex-Playboy model Karen McDougal, who had accusations of an affair; and to porn performer Stormy Daniels, who alleged a sexual encounter with Trump. He has pleaded not guilty and says the stories are all fake.
The trial, the first of Trump's four criminal cases to come before a jury, is expected to last for another month or more. And with every moment Trump is in court, he's growing increasingly frustrated while the November election moves ever closer.
The Associated Press contributed to this report.
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