Manganese (Mn) pollution in aquatic environments poses threats to fish health, yet the molecular mechanisms of Mn-induced cardiac injury remain poorly understood. Epigallocatechin-3-gallate (EGCG), the major bioactive catechin in green tea, exhibits potent antioxidant and anti-inflammatory properties. Bioinformatics revealed overlap between Mn toxicity targets and EGCG-modulated genes, enriched in oxidative stress, ER stress and inflammation-related mechanisms. Therefore, this study aimed to investigate the molecular mechanisms by which Mn exposure induces cardiotoxicity in common carp and to evaluate whether dietary EGCG supplementation confers protection against cardiac injury, with particular focus on the roles of integrated stress response (ISR) and inflammation. Carp were exposed to Mn²⁺ with or without dietary EGCG. We found that Mn exposure resulted in myocardial histological structural damage, elevated serum cardiac injury markers, and reduced myocardial ATPase activities, indicating cardiac injury at the structural and biochemical levels. Mechanistically, Mn-induced oxidative stress (decreased SOD1, CAT, NRF2; increased MDA) was associated with ER stress and ISR-related transcript changes (upregulated HSPA5, DDIT3, eiF2AK4, atf4b) and triggered inflammatory responses (increased IL-1β, IL-6, IFN-γ, iNOS/NO), alongside upregulated inflammatory signaling nodes (NF-κB, STAT3, JUN, MAPK1, MAPK14b). EGCG supplementation significantly reversed these pathological changes. Importantly, ISRIB treatment in EPC epithelial cells and H9c2 rat cardiac-derived myoblasts attenuated Mn-induced inflammatory gene expression, providing supporting mechanistic evidence that ISR-associated signaling may contribute to inflammation, although these in vitro findings do not directly confirm the proposed mechanism in carp cardiomyocytes. Taken together, these findings support a proposed mechanistic framework in which waterborne Mn exposure is associated with oxidative stress, ER stress, and ISR-related signaling, together with inflammatory responses and cardiac injury in common carp, rather than a conclusively demonstrated causal pathway. Dietary EGCG supplementation attenuated these Mn-associated responses and was associated with reduced cardiac injury. Our work provide a new mechanistic way of thinking for understanding heavy metal-caused cardiac injury in fish and improving the current application potential of EGCG as an experimental dietary intervention for aquatic pollution stress management.