We here compare the tropical arthropod fauna across a freshwater swamp and six different forest types (rain-, swamp, dry-coastal, urban, freshwater swamp, mangroves) based on 140,000 specimens belonging to ca. 8,500 species. Surprisingly, we find that mangroves, a globally imperiled habitat that had been expected to be species-poor for insects, are an overlooked hotspot for insect diversity despite having low plant diversity. Mangroves are very species-rich (>3,000 species) and distinct (>50% of species are mangrove-specific) with high species turnover across Southeast and East Asia. Overall, plant diversity is a good predictor for insect diversity for most habitats, but mangroves compensate for the low number of phytophagous and fungivorous species by supporting an unusually rich community of predators whose larvae feed in the productive mudflats. For the remaining habitats, the insect communities have diversity patterns that are largely congruent across guilds. The discovery of such a sizeable and distinct insect fauna in a globally threatened habitat underlines how little is known about global insect biodiversity.