Ibrahim al-Maamari, editor of Azaman daily, was arrested on July 28, two days after it published an article alleging that government officials tried to have judicial authorities change a 2015 ruling to the benefit of certain influential figures, Reporters Without Borders (RSF) said.
"We condemn Ibrahim al-Maamari's arrest and continuing detention," Alexandra El Khazen, RSF's Middle East chief, said in a statement.
"By punishing this journalist in this way and by keeping him in detention, the Sultanate of Oman is sending a negative message about media freedom and is demonstrating an inability to tolerate criticism of the political and judicial system," she said.
"We call for an independent and impartial investigation and for Al-Maamari's immediate release," she added.
RSF cited a spokesman for the public prosecutor's office as saying the article was considered a "public crime" that discredited the integrity of the judiciary and government officials.
He said Maamari was arrested following several complaints.
"By publishing false information, (Maamari) had violated articles 25 and 29 of the Press and Publications Law, which is punishable by up to two years in prison and a fine," RSF cited the prosecutor's office as saying.
He is also accused of "undermining the prestige of the state" under an article of the penal code and publishing news that would disturb public order, it added.
Azaman on Tuesday left half of its front page blank in protest after the information ministry banned it from publishing a story on Maamari's arrest, RSF said.
RSF recalled that Maamari was sentenced to five months in prison in September 2011 and the paper banned for a month due to an article deemed insulting to the justice minister.
The sentence was quashed when the paper published an apology, RSF said.
Ibrahim al-Maamari, editor of Azaman daily, was arrested on July 28.
Media watchdog Reporters Without Borders on Tuesday condemned Oman's detention of a newspaper editor over an article accusing officials of pressuring the judiciary in an inheritance case.
