Resumo. Abstract: Aim This study investigated whether seasonal variations have an effect on diet composition of Serrapinnus notomelas in a marginal lagoon located under the area of influence of the Tibagi River, Upper Paraná Basin. Methods Samples were carried out monthly between February 2017 and January 2018, and fish specimens were caught with two sieves (2 mm mesh). The volumetric and occurrence method were used to quantify stomach contents. Results A total of 358 stomachs had their contents analyzed. The autochthonous resources were dominant in the diet of S. notomelas in all seasons, with a predominance of algae, detritus and Tecamebas. Meanwhile, plant material was the most abundant allochthonous resource in the diet. Diet composition showed significant differences between all seasons, while in the autumn and summer seasons, individuals showed greater trophic niche breadth, respectively. Conclusions Our results show the great importance of autochthonous resources for the maintenance of the S. notomelas population, and that seasonality can influence the trophic composition in the species' diet. We observed difference in the diet during the seasons and that during autumn and summer, individuals presented a greater breadth of the trophic niche. Thus, our results corroborate with knowledge to the preservation of small fish species, which are fundamental for the trophic network in ecosystems.