The plastic debris contamination, feeding ecology and habitat use of Cynoscion acoupa, an economically important species, were studied in relation to spatial, seasonal and ontogenetic aspects in a tropical estuary. The human impact on the environment was evident in the studied species, reflecting the high contamination of the diet of these fish with plastic debris. Plastic debris is frequently ingested by C. acoupa, regardless of season, area or ontogenetic phase. However, a higher frequency and amount of ingestion suggests that this contaminant might also be acquired through direct consumption and biotransference (from contaminated prey). The ontogenetic phases of C. acoupa utilized different estuarine reaches during seasonal fluctuations. This behaviour was strongly influenced by environmental variables, particularly salinity. During all seasons, juveniles used the upper estuary as shelter to avoid marine predators. However, in the early rainy season, the upper estuary was significantly important as a nursery for this species. The sub-adults used the upper estuary as a feeding ground during the year and migrated to the middle estuary during the late rainy season to avoid osmoregulatory stress. Adults were observed only in the coastal waters of the lower estuary. The trophic guild changed during the ontogeny of C. acoupa, and juveniles primarily fed on plastic debris (FO = 64%), amphipoda (FO = 34%), mysidacea (FO = 17%) and Cathorops spixii (FO = 15%). Sub-adults preferred plastic debris (FO = 50%) and C. spixii (FO = 30%). Both juveniles and sub-adults were classified as opportunistic. Adults were piscivorous, ingesting plastic debris (FO = 100%), C. spixii (FO = 18%), Achirus lineatus (FO = 15%), Stellifer stellifer (FO = 15%) and penaeid shrimp (FO = 15%).