Local and regional species diversity is influenced by historical factors, resource availability, biotic interactions, the presence of resources, and adequate environmental conditions. This study investigated the influence of local environmental variables such as temperature, depth, turbidity, and dissolved oxygen on fish richness and composition along the beaches of the Acre River in Brazil. The sampling period extended from June to September 2017, encompassing 31 sandy beaches. A comprehensive collection effort yielded 15,627 individuals representing 60 distinct fish species. The PCA indicated that beaches located in the upper part of the river tended to have higher pH, current speed, and shallower depth, while the lower part had greater length, warmer waters, and greater concentration dissolved oxygen. There was a trend towards lower species richness in the upper part and greater richness in intermediate portions of the Acre River. Multiple regression indicated that only water temperature influenced positively the variation in fish species richness as predicted by the energy-environment hypothesis. Also, water temperature, concentration of dissolved oxygen, and depth were essential to determine changes in species composition. Therefore, local environmental variables, especially the water temperature, are important factors in determining the fish richness and variation in species composition on sandy beaches of the Acre River.