RSF said Souleymane Salha, director of the weekly Le Courrier, had been in custody in the capital Niamey since Monday over the publication of an article in which he criticised the deputy police chief over the arrest of opposition presidential candidate Hama Amadou.
Amadou, a former prime minister who fled the country to escape baby trafficking charges, was detained on November 14 as he returned after a year in exile in France.
Two journalists and two cameramen covering Amadou's return on November 14 were arrested and their footage confiscated, RSF said, adding that they were freed the next day.
In January, around a dozen of Niger's radio and television stations simultaneously suspended broadcasting for five minutes and aired an on-screen message that read "I Am Freedom of the Press", in a protest over police violence against reporters.