Heavy metal contamination has emerged as a major environmental challenge threatening aquatic ecosystems and human health worldwide. Rapid industrialization, urbanization, mining activities, agricultural intensification, and improper waste disposal have significantly increased the release of toxic metals into aquatic environments. Heavy metals such as mercury (Hg), cadmium (Cd), lead (Pb), arsenic (As), chromium (Cr), nickel (Ni), copper (Cu), and zinc (Zn) enter water bodies through natural and anthropogenic pathways and subsequently accumulate in water, sediments, and aquatic organisms. Due to their persistence and non-biodegradable nature, these contaminants undergo bioaccumulation within organisms and biomagnification across trophic levels, resulting in elevated concentrations in predatory fish and other higher consumers. Exposure to heavy metals adversely affects aquatic organisms by impairing growth, reproduction, metabolism, immune function, and behavior, while also causing histopathological alterations, oxidative stress, genotoxicity, endocrine disruption, and cellular damage. At the ecosystem level, heavy-metal contamination contributes to biodiversity loss, altered community structure, disrupted food-web interactions, and reduced ecosystem productivity. Human exposure occurs primarily through the consumption of contaminated fish and seafood, posing risks of neurotoxicity, renal dysfunction, developmental abnormalities, carcinogenicity, and other chronic health disorders. Future research should focus on the interactions of heavy metals with emerging pollutants, climate change influences, omics-based biomarkers, and sustainable remediation technologies. Effective monitoring, pollution control, and integrated management strategies are essential for safeguarding aquatic biodiversity, ecosystem health, and food security.