Petar Komnenic, who had alleged that authorities in Montenegro placed a number of senior judges under illegal surveillance, was convicted of libel in February 2011 and ordered to pay a fine or serve four months in jail.
Komnenic refused to pay the fine and appealed to a higher court, which ruled he should serve community service rather than a prison sentence.
But the Podgorica magistrate on April 18 reinstated the prison term.
Komnenic is an editor with Podgorica-based TV Vijesti and also works as a stringer for RFE/RL and Reuters.
The European Union has expressed concerns over the case.
Montenegro is scheduled to start EU accession talks in June this year.