Masoud Haidari was managing director of Iran's labour news agency ILNA, which had often reported on workers' protests and arrests of rights activists. It was closed by the authorities in July last year and recently reopened under new management.
Analysts saw the closure as part of a broader crackdown on dissenting voices when the Islamic Republic is facing international pressure over its disputed nuclear programme. The West says Iran wants atomic bombs, which Tehran denies.
Iran says it allows free speech. Iranian journalists say they have to tread carefully in their reporting to avoid having their publications closed.
Haidari was sentenced to three months and a day in prison "for publishing lies" and fined 1,500,000 rials (80 pounds) "for defamation", the students news agency ISNA said. The fine was in lieu of lashes.
The plaintiffs included a university and a senior official in Iran's Labour Ministry, ISNA reported. Haidari can appeal.
(Reporting by Hossein Jaseb, writing by Edmund Blair)
