Deforestation is a major threat to biodiversity in tropical streams. Intact riparian forests may play an important role in mitigating the effects of deforestation on tropical streams, but few studies have quantified the impacts of riparian buffers on tropical stream communities. We investigated the influence of riparian forest buffers on stream macroinvertebrate communities in southeastern Costa Rica. Macroinvertebrates were collected from 12 sites in 9 small, lowland streams in 2005 and 2006. Sites were divided evenly among the following three treatments: forested reference stream reaches (FR), stream reaches in pastures with riparian forest buffers (RB) that met the requirements of Costa Rican law, and stream reaches in pastures (P) without riparian buffers. Total taxa richness and EPT taxa richness were significantly higher in FR than in P sites. Both measures were intermediate in RB sites, which did not differ significantly from other treatments. Ordination of sites using Non-metric Multidimensional Scaling (NMS) showed clear differences in macroinvertebrate community composition between FR and P sites. RB sites generally grouped with FR sites. Our data suggest that riparian forest buffers can reduce the impacts of deforestation on tropical stream communities, and provide support for existing Costa Rican legislation protecting riparian forests.