Questions: What is the species composition and density of the seed bank in one of the most arid deserts of the world: the Atacama Desert? Do composition and density vary along the aridity gradient? Are they affected by the sampling season and soil nutrient content? Location: Southern Atacama coastal desert, Chile. Methods: Seed bank composition and density were estimated via direct counting of seeds extracted from soil samples collected in eight localities separated by 0.5°, between 26-30° S along the study geographic gradient. This sampling protocol was conducted at three different seasons: after seed set and before and after germination in the rainy season. We also estimated soil nutrient content and obtained local climatic data from the WorldClim database to perform an ordination of seed banks along the aridity gradient. Results: The entire seed bank was composed of 89 species (38 endemic), including 51 annuals (20 endemic). Annuals represented 82.4% of all seeds counted. Exotic plants were represented by 11 species, including nine annuals, with higher seed densities towards the more mesic localities. Aridity was by far the main factor driving seed bank composition and seed density along the study gradient. Seed banks were similar among seasons across all localities. Soil nutrient content played an important, but secondary, role in species-specific relationships. In comparison to other ecosystems, the seed bank densities found in this study correspond to the lowest figures reported to date. Conclusions: This study represents the first report on seed banks via direct counting for the southern part of the Atacama coastal desert. The level of aridity mainly determines annual plant communities in this desert, including exotic species. Their responses to the aridity level and soil nutrient content are species-specific, which allows for species co-existence under extreme abiotic conditions.