Groundwater influences stream environments in numerous ways including structuring biotic assemblages. However, associations between groundwater influence and warmwater fish assemblages are under-studied. We examined relationships between groundwater contribution, population size, and total length (TL) for 5 warmwater fishes at 32 stream reaches in the Ozark Highlands ecoregion. When we controlled for distance from an impoundment, population size and TL were significantly related to groundwater influence for all 5 species. Sunfishes were significantly less abundant in reaches with high levels of groundwater contribution (HGC reaches), whereas Ambloplites rupestris (Rock Bass) and Nocomis asper (Redspot Chub) TLs were significantly greater at HGC reaches. Reach-scale groundwater contribution explained nearly 4 times more unexplained variation among fish densities than did TL. Our study provides insight into the structuring role of groundwater on warmwater fish populations.