Indigenous land rights have been increasingly recognized in several countries, but powerful economic and political lobbies have conspired to produce major legislative alterations to severely weaken the protection status of indigenous territories, threatening their rich ethnocultural capital and biological diversity. Here, we provide a comprehensive quantitative assessment of the demography of indigenous populations and the conservation performance of all 587 physically demarcated Indigenous Lands (ILs) in Brazil. Our results show that human population density (HPD) is higher inside than outside 50 % of ILs, dispelling the often repeated argument that there is “too much land for too few Indians”. Moreover, a strong positive relationship between IL size and indigenous population size is corroborated by larger resident populations within the largest territories, even they are sparsely settled. Over half of all ILs retain 90 % of natural vegetation and harbour 54 % of all indigenous populations living inside ILs. HPD within ILs was strongly negatively related to their proportion of natural vegetation cover. These results show the critical importance of legally protecting sufficiently large indigenous territories. Any alteration in the protection status and/or opening up ILs to economic exploitation such as mining, forestry and large-scale agriculture will affect the long-term ethnocultural integrity and the environmental viability of these territories. ILs remain critical if Brazil is to accomplish its international commitments to both protect tropical biodiversity and mitigate climate change.