Abstract Aim We examine the influence of geography on species richness and endemism of mammals on tropical Pacific archipelagos to determine the importance of intra- and inter-archipelago speciation in promoting local and regional species richness. Location Thirty tropical Pacific archipelagos. Methods A distributional list of mammals on 30 archipelagos was compiled, and values for 10 geographical variables were estimated for each archipelago. Mammal species were placed in three different categories (continental, Pacific and endemic) based on their distribution. The total number of species and numbers of species within each category were related to the geographical variables using Poisson regression analysis. Results Species richness was related positively to variables describing land area, numbers of large islands and elevation; and negatively to variables describing isolation. Levels of endemism did not differ between volant and non-volant species, but differed between mega- and microchiropterans. Main conclusions Variation in species richness of mammals in the tropical Pacific region can be accounted for by a combination of intra-archipelago speciation within archipelagos composed of large islands, and inter-archipelago speciation, particularly among more isolated archipelagos. Mammals were less widely distributed throughout the study area than previously found for butterflies, skinks or birds. However, the level of endemism was similar to that of skinks and birds on the same archipelagos, and was higher than that of butterflies.