In 2018 the Inter-American Court of Human Rights delivered the first—and so far only—judgment against Brazil concerning indigenous land rights. The court’s historic decision ruled against the state, highlighting the state’s failure to remove non-indigenous individuals from the territory of the Xukuru people. At issue before the court was the demarcation of Xukuru’s land, which can be regarded as a standard step in South-American judicial-administrative procedures for recognizing and protecting indigenous peoples’ rights to their traditional lands, in which anthropologists participate as experts. The international litigation proceedings counted on the expert testimony of anthropologist Christian Teófilo da Silva, a professor at a prestigious Brazilian university. Interestingly, the anthropologist Vânia Rocha Fialho de Paiva e Souza participated in the international adjudicatory process as a representative of the victims and cross-examined her colleague. Anthropologists and expert evidence played multiple roles in protecting the Xukuru territory. By analyzing ethnographical material (Rocha Fialho de Paiva e Souza 1992), court documents and the recorded oral expert witnessing, this paper addresses the question of what the role of anthropologists in advancing the protection of indigenous lands rights was. In doing so, it articulates the Brazilian and Inter-American legal framework on indigenous land rights in light of the literature on anthropological and cultural expertise (Foblets 2016; Holden 2020). As a result, it underscores the evolving challenges regarding the dialogue between law and anthropology in courtroom settings.