Intensive fisheries have reduced fish biodiversity and abundance in aquatic ecosystems worldwide. ‘No-take’ marine reserves have become a cornerstone of marine ecosystem-based fisheries management. and their benefits for adjacent fisheries are maximized when reserve design fosters synergies among nearby reserves. The applicability of this marine reserve network paradigm to riverine biodiversity and inland fisheries remains largely untested. Here we show that reserves created by 23 separate communities in Thailand’s Salween basin have markedly increased fish richness, density, and biomass relative to adjacent areas. Moreover, key correlates of the success of protected areas in marine ecosystems—particularly reserve size and enforcement—predict differences in ecological benefits among riverine reserves. Occupying a central position in the network confers additional gains, underscoring the importance of connectivity within dendritic river systems. The emergence of network-based benefits is remarkable given that these reserves are young (less than 25 years old) and arose without formal coordination. Freshwater ecosystems are under-represented among the world’s protected areas, and our findings suggest that networks of small, community-based reserves offer a generalizable model for protecting biodiversity and augmenting fisheries as the world’s rivers face unprecedented pressures.