The order was passed by a Delhi court.
Narmada Bachao Andolan founder Medha Patkar has been convicted in a criminal defamation case filed against her by Delhi Lieutenant Governor VK Saxena. Ms Patkar faces a fine or a jail term of two years, or both.
The case had been filed in 2006 and was being heard by a Delhi court. The conviction order was passed by Metropolitan Magistrate Raghav Sharma on Friday.
Magistrate Sharma said that reputation is one of the most valuable assets a person can possess, as it affects both personal and professional relationships and can also significantly impact an individual's standing in society.
The arguments on the quantum of sentence will be heard on May 30.
Ms Patkar and Mr Saxena's legal battle dates back to 2000, when the lieutenant governor was the chief of the National Council for Civil Liberties, an Ahmedabad-based NGO. The activist had filed a case against him for publishing advertisements against her and the Narmada Bachao Andolan.
Mr Saxena had also filed two cases against the activist for making "derogatory" remarks about him on a TV channel in 2006 and for issuing a "defamatory" press statement.
The Narmada Bachao Andolan had claimed that the construction of the Sardar Sarovar Dam in Gujarat, which was inaugurated in 2017, could affect 40,000 families. It had organised massive protests, stating that the families could have to leave their homes, which could be submerged.
The project had been embroiled in controversies since its foundation stone was laid in 1961 by then PM Jawaharlal Nehru.
(With agency inputs)