Deng Athuai, chairman of the South Sudan Civil Society Alliance, was kidnapped July 4 in Juba, the capital of South Sudan. The advocacy group said he was detained for three days and beaten by assailants seeking information about his anti-corruption work in the country.
Dana Wilkins, a campaigner for Global Witness, called on South Sudan's government to make good on pledges to bring Athuai's assailants to justice.
"When civil society's freedom to operate is threatened, all efforts to fight corruption and impunity are threatened," said Wilkins in a statement.
South Sudan gained independence in July as part of a peace deal that ended the Sudanese civil war, one of the bloodiest conflicts in modern history.
The country is one of the poorest in the world, however, and revenues were curtailed after oil production was halted during a January dispute with Sudan.
South Sudanese President Salva Kiir, in a June letter to civil servants, said an estimated $4 billion was missing from federal coffers. Some of the money, he said, was likely deposited in foreign accounts by officials suspected of corrupt practices in the country.