The international branch of the human rights group Memorial says Russia's Justice Ministry has designated it as a "foreign agent."
The group said on its Facebook account on October 4 that it considers the move illegal, and that it intends to appeal against it in court.
Another branch of the widely respected Moscow-based organization, the Memorial Human Rights Center, was placed on the government's register of "foreign agents" in November 2015.
In September, the Justice Ministry placed independent pollster Levada Center on the register, potentially threatening its existence.
A 2012 law requires any NGO that receives funding from abroad and engages in political activity to formally register as a "foreign agent."
Russian and international human rights organizations have said the law was introduced to silence independent voices.
Amendments introduced to the law in 2014 allow the Justice Ministry to unilaterally add NGOs to the list of "foreign agents." Failure to comply can result in heavy fines and jail time.