Noureddine Boutar, head of Mosaique FM, was arrested at his home in the capital Tunis on Monday and taken to the counter-terrorism brigade’s headquarters in the district of El Gorjani.
Boutar’s lawyer, Dalila Ben Mbarek Msadek, said the authorities had provided no explanation for Boutar’s detention.
RSF said Boutar’s arrest was "a forceful message to the media from the authorities".
Khaled Drareni, RSF’s North Africa representative, said Tunisia’s goal was "to terrorise and subjugate journalists, and to send them back to the era of the Ben Ali dictatorship".
"This growing authoritarianism, previously signalled and now amply confirmed, must be strongly condemned, and Noureddine Boutar must be released at once."
Boutar’s detention came amid a spate of arrests of political activists, former judges and a prominent businessman this week.
The arrests are a continuation of the oppressive rule of Tunisian President Kais Saied, under which opposition figures, lawyers and journalists have been arrested.
Saied, who became Tunisian president in 2019, froze parliament and seized far-reaching executive powers in July 2021 in what critics have called a "coup" and an attack on the only democracy to have emerged from the Arab Spring uprisings more than a decade ago.
Saied later took control of the judiciary and pushed through a new constitution, giving his office almost unlimited powers.
Mosaique FM, which was launched in 2003, expressed "astonishment" at Boutar’s arrest and denounced "the smear campaign and intimidation" the radio station has been subjected to.
Other organisations also condemned the radio director’s arrest. The Tunisian Federation of Newspaper Directors (FTDJ) called it ”concerning" for the status of the media and communications sector in Tunisia, while The National Union of Tunisian Journalists (SNJT) organised "a day of anger" sit-in outside the government headquarters on Thursday to protest the Tunisian state’s latest attacks on press freedom.
A clampdown on press saw him criminalise "fake news and rumours” in September 2022.