AbstractAquatic mammals play an important role in community structure. The present study applied stable isotope analysis (SIA) to evidence trophic relationships and resource partitioning among aquatic mammals inhabiting different environments in the Amazon estuarine complex and adjacent coastal zone (AE) and Northeastern coast (NC) of Brazil. In addition, isotopic niche partitioning among Sotalia guianensis, Inia spp. and Trichechus inunguis within the AE was also evaluated, and ecological S. guianensis stocks were characterized. Among marine delphinids, the carbon isotopic composition in offshore species mirrored that of nearshore species, contradicting the pattern of decreasing δ¹³C values characteristic of many areas around the world including areas in Southeastern and Southern Brazil. Isotopic niches were highly distinct, with no overlap among the assessed species inhabiting the AE. Inia spp. and T. inunguis occupied significantly larger isotopic niche spaces, suggesting high habitat plasticity. S. guianensis inhabited two coastal regions indicating an ecological distinction. Nitrogen values were similar between S. guianensis from the NC and AE, indicating comparable trophic positions. However, NC specimens presented more variable δ¹³C values compared to those from AE. SIA results also allowed for insights concerning habitat use and the trophic ecology of dolphin species inhabiting different oceanographic regions off Northern/Northeast Brazil. These findings provide novel data on the stable isotope composition for cetaceans and sirenians from this region, and aid in furthering knowledge on the trophic ecology and habitat use of the investigated species.