Tropical forests occurring on white-sand soils have a unique structure and are famous for their endemism. Yet, no comprehensive floristic study has ever been undertaken in white-sand forests in the western Amazon. Here, we present the results of floristic inventories from 16 plots in seven sites from the Peruvian Amazon to investigate diversity, species composition, and endemism in white-sand forests. We compare our results to a large data set from terra firme forests from more fertile soils in the same region. We found that white-sand forest plots have extremely low average species diversity (41.5 species per 0.1-ha plot) and that white-sand plots have significantly different species composition from terra firme plots. We classify 114 species as endemic to white sand, with another 21 species that can be considered facultative specialists or cryptic endemics. These endemics and specialists are extremely dominant, accounting for more than 83% of the total number of stems surveyed in white-sand forest plots. We place our results in the context of the role of environmental heterogeneity influencing patterns of species diversity and the conservation of Amazonian forests.