Donald J. Trump was chastised on Tuesday by the judge presiding over his civil fraud on Tuesday trial after the former president posted a message targeting the judge’s law clerk.

Mr. Trump attacked the clerk, Allison Greenfield, shortly before noon on his Truth Social site. His post was a picture of Ms. Greenfield with Senator Chuck Schumer, the Democratic majority leader. Mr. Trump mocked Ms. Greenfield as “Schumer’s girlfriend” and saying that the case against him should be dismissed.

The post was taken down during a lunch break, around 1:30 p.m.

Justice Arthur F. Engoron, after the break, explained what had happened, though he did not name Ms. Greenfield or Mr. Trump, referring to him only as a defendant. “Personal attacks on my members of my court staff are unacceptable, inappropriate and I will not tolerate them under any circumstances,” he said.

Justice Engoron said that his statement should be considered a gag order forbidding any posts, emails or public remarks about members of his staff.

He added that serious sanctions would follow were he to be disobeyed.