The next time the 80-year-old retired chemistry professor takes his protest to the plaza outside the federal courthouse in Manhattan, he may make it home without being locked up.

A federal judge on Thursday ordered the dismissal of an indictment against the professor, Julian P. Heicklen, who had been charged with jury tampering for advocating the controversial position known as jury nullification while outside the courthouse.

Mr. Heicklen had repeatedly stood with a “Jury Info” sign and handed out brochures supporting nullification, the view that jurors who disagree with a law may ignore their oaths and vote to acquit a defendant accused of violating it.

Prosecutors said such advocacy, “directed as it is to jurors, would be both criminal and without constitutional protections no matter where it occurred.”