Mercury (Hg) is considered one of the most dangerous toxic metals for the environment and human health. The present study aimed to investigate mercury exposure in riverside communities of the Lower and Middle Tapajós River. This is an observational, cross-sectional, descriptive, and quantitative study, including 126 individuals from eight communities, carried out between 2021 and 2022. A questionnaire was administered, and hair samples were collected to measure total Hg levels. Results showed that Hg levels exceeded the World Health Organization (WHO) recommended exposure limit of 2 µg/g in all the communities, and in 95% of the participants. The highest levels of Hg were observed in two individuals from the São Luís do Tapajós community (18.60 µg/g). In relation to the symptoms reported, “muscle and joint pain” was mentioned by participants from all the communities and “decreased motor coordination” in four of the communities visited. These results indicate that participants from communities in the study region are exposed to Hg and the levels observed are sufficient to trigger the reported symptoms. Results from this study can influence public policies aimed at curbing illegal activities that increase Hg contamination of aquatic ecosystems and aquatic biota, including key species for the food security of riverine populations in the Amazon region.