Most eukaryotic organisms are arthropods. Yet, their diversity in rich terrestrial ecosystems is still unknown. Here we produce tangible estimates of the total species richness of arthropods in a tropical rainforest. Using a comprehensive range of structured protocols, we sampled the phylogenetic breadth of arthropod taxa from the soil to the forest canopy in the San Lorenzo forest, Panama. We collected 6,144 arthropod species from 0.48 ha and extrapolated total species richness to larger areas based on competing models. The whole 6,000 ha forest reserve most likely sustains 25,000 arthropod species. Remarkably, just 1 ha of rainforest yields > 60% of the arthropod biodiversity held in the wider landscape. Models based on plant diversity fitted the accumulated species richness of both herbivore and non-herbivore taxa exceptionally well. This lends credence to global estimates of arthropod biodiversity developed from plant models.