In the last twenty years, independent national institutions mandated to protect and promote human rights (NHRIs) have changed the global human rights architecture, positioning themselves as indispensable interlocutors when discussing states’ human rights records. Nevertheless, despite having a broad mandate, comparative experiences have shown that NHRIs rarely exploit the possibility to oversee the security sector. Academic literature on NHRIs also provides little information on their role in oversight of the security sector in general, let alone the security services. That is surprising, considering that actions of the security services may interfere with human rights in unparalleled way, as they are authorized to use special measures to penetrate deep into the private lives of citizens. This article seeks to contribute to the mainstreaming of the role of NHRIs in security services oversight by further exploring their unique mandate, added value and comparative advantages. It shows that the specific nature of the NHRI makes this institution well placed to make a substantial contribution to effective protection of the rights and freedoms of citizens affected by activities of the security services. We start with positioning NHRIs in the security sector and defining their general role in it. We then offer a comparative overview of powers and practices of NHRIs and lay down preconditions for NHRIs’ effective actorness in security sector oversight. We then move to a case study of the Serbian Ombudsman, as a practical exemplar of the role of NHRIs in security services oversight. We conclude with an overview of the results, their practical implications and the avenues for further research.