This study assessed heavy metal (HM) concentrations and associated ecological and human health risks in sediments from the Rezu Khal seaweed culture site, Cox’s Bazar, Bangladesh. Analyzed heavy metals (HMs) included Pb, Cd, Cr, Co, Ni, Cu, Zn, Fe, and Mn, with concentrations following the decreasing order: Fe > Mn > Zn > Ni > Cr > Pb > Cu > Co > Cd. Strong positive correlations (r > 0.67, p < 0.05) were observed among Pb, Cu, Zn, Fe, Co, and Cr, suggesting potential common sources. The geo-accumulation index (Igeo) classified most metals as “practically uncontaminated,” except Cd, which showed “moderate contamination,” and Ni, ranging from “uncontaminated to moderately contaminated”. The enrichment factor (EF) analysis further confirmed Cd and Ni as the most enriched metals. Pollution load index (PLI) values (0.708 to 1.068) fluctuated around the critical threshold of 1, indicating minimal contamination. Potential ecological risk index (PERI) values (107.87 to 133.31) suggested a “low ecological risk,” with Cd being the primary contributor. Human health risk assessment (HHRA) revealed higher non-carcinogenic risk (NCR) for children via Co ingestion, though the hazard index (HI) remained below 1 for both children and adults. Carcinogenic risk (CR) assessment indicated Pb and Cd were within acceptable limits, while Cr exceeded the threshold for children via ingestion, posing a significant cancer risk for them. These findings highlight that the region is mildly contaminated and does not require immediate mitigation strategies, however, particular attention should be given to Cd and Ni. Further research on long-term trends and anthropogenic impacts is essential to ensure sustainable environmental management at the site.