The presence of potentially toxic metals in aquatic environments is a growing concern, particularly in urban and industrialized regions where anthropogenic activities intensify contamination. Metals such as Pb, Cd, and Hg are especially relevant due to their persistence, bioaccumulation potential, and adverse effects on aquatic ecosystems and human health. This study quantified Pb, Cd, and Hg concentrations in water, sediments, and fish from an urban river segment in Santo André, São Paulo State, Brazil. To date, no systematic monitoring of metal contamination has been reported for this river, highlighting the novelty of this biomonitoring assessment. Surface water samples were collected at three sites spaced approximately 200m apart, and sediment samples were obtained at two intermediate locations. Sampling was conducted during the rainy (February) and dry (August) seasons to evaluate seasonal variability. Fish were collected during the rainy period; no specimens were found during the dry season due to a previously reported mass mortality event. Metal accumulation in fish was assessed separately in muscle and skin tissues. Metal concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS). In surface water, metal concentrations reached up to 7 ppb, whereas sediment concentrations reached values as high as 10,000 ppb. Pb and Cd were detected in all matrices, while Hg was detected only in sediments and fish tissues. Higher concentrations were observed in sediments and biota compared to the water column, indicating accumulation processes and potential trophic transfer. Metal concentrations increased by at least 20% during the dry period, and analysis of variance (ANOVA) confirmed significant seasonal differences. These results indicate concerning levels of metal contamination in this metropolitan river system, highlighting potential ecological risks and human health implications related to water use and fish consumption. This study provides essential baseline data for future environmental monitoring in urban tropical rivers.