In 2014, the United Nations established a working group to elaborate an international treaty on business and human rights. In October 2018, negotiations on a first draft of the actually text took place. Besides this zero-draft, the working group released the draft text of an Optional Protocol containing several institutional arrangements. The Optional Protocol carves out a key role for national implementation mechanisms to promote compliance with, monitor and implement the treaty on business and human rights. With such an institutional arrangement, the future treaty would join the ranks of what can be called a new generation of human right treaties which institutionalize a top down with a bottom up approach aiming to address the disjuncture between rules and practice. The Optional Protocol indicates that this role of national implementation mechanism could be taken up by National Human Rights Institutions (NHRIs). This follows an increased recognition of NHRIs as significant actors in the business and human rights domain. Yet, the role of NHRIs in human rights governance in general, and in the business and human rights field in particular, is not yet well understood and undertheorized. The aim of this article is to add insight into the role of NHRIs in business and human rights by, first, describing some of the current activities undertaken by NHRIs in this field in order to analyze whether the role ascribed to them is actually being taken up and what challenges NHRIs face. From this perspective, it will be discussed whether the role foreseen in the Optional Protocol of the future business and human rights treaty holds promise.