Reporters Without Borders (RSF) in a statement on Tuesday said Muhammad al-Amin walad Mahmudi, a freelance journalist, was arrested on Sunday.
RSF also condemned his detention. "The violation of the right to information once again shows the repressive will of the Mauritanian authorities, who, embarrassed by a message, send in the police to punish the messenger."
Slavery, though practised, is an issue widely seen as taboo in Mauritania.
Mauritanian officials confirmed the arrest but declined to comment further. The detained journalist's lawyer, Bah walad al-Bar said the case had gone before a regional court but was unlikely to be resolved for several days as the prosecutor was absent.
It was not immediately clear what charges the journalist might face.
"The violation of the right to information once again shows the repressive will of the Mauritanian authorities, who, embarrassed by a message, send in the police to punish the messenger"
Reporters Without Borders
Slavery was officially abolished in 1981 in Mauritania but is still practised in some areas, where children born into former slave families labour as forced domestic servants and farm workers without payment, rights groups say.
Walad Mahmudi was seized along with the wife of a senior opposition official and a teacher while they were researching a story on slavery near the town of Mederdra, in the south of the West African country, RSF said.
The journalist had interviewed a young woman alleged to have been in forced labour who fled after being mistreated.
But after reporting her case to the police, she was allegedly returned to her masters, according to local anti-slavery group SOS Esclaves.