Our planet is undergoing the sixth mass extinction of species. The accelerated change in land use coupled with other threats to biodiversity, has led to a worrying process of defaunation. For this reason, it is necessary to know the current state of those groups and species that are at greater risk of extinction or that require studies, in order to take urgent conservation measures, particularly in regions such as South America, where the loss of natural habitats is alarming. We assess the population trends and conservation status of South American mammals. We used the IUCN databases for 1197 species. Analyses were carried out at the species and order level. The results in terms of population trend indicate that of the total analysed, almost 30% are declining, and less than 1% are increasing, while there was no information of 46% of the species. The orders with most critical declining species are Carnivora, Primates, Chiroptera, Rodentia and Cetartiodactyla. In relation to the IUCN conservation status, analyses indicate that 16% of the species are under some category of risk. For example, of 44 species of carnivores, 9 have some category of danger; in the case of the primates, of the 130 species 57 are in some category of danger. This analysis shows that a third of the mammals in South America are at risk of disappearing. This percentage is likely to be higher if one takes into account that there is no information on population trends of nearly half of the species.