Osinmo Reservoir is an important water body in south-western Nigeria. Despite being critically important, there is a lack of detailed biological studies on this reservoir. This study investigated the effect of seasonal variations on the characteristics of the sediment and the community structure of the macroinvertebrates in the reservoir between May 2012 and March 2014. The sediment was generally slightly acidic and was dominated by silty sand and sandy mud with low organic matter. Twenty-four taxa of macroinvertebrates with a total abundance of 55,065 individuals (ind.) belonging to three phyla were recorded. Overall, the composition and the abundance of the macroinvertebrates were not significantly different (P > 0.05) in the wet and dry seasons. Seasonal variations in the sediment quality did not have a significant impact on the community metrics such as species diversity, species richness and species evenness. The dominance of sensitive species indicated a moderately clean status of the reservoir.