<p>This study aimed to investigate the effects of dietary omega-3 fatty acids on cardiac and liver function, mortality rates, hematological parameters, tissue morphometric, and gene expression in broilers exposed to high stocking density (HSD). A total of 420 one-day-old Arian broiler chicks were allocated to a 2 × 2 factorial arrangements consisting of two stocking densities [standard stocking density (SSD; 9 birds/m²) and high stocking density (HSD; 16 birds/m²)] and two levels of omega-3 supplementation (0.057% basal vs. 0.5%) within a completely randomized design. At 40 days of age, birds reared under HSD and supplemented with 0.5% omega-3 showed significantly lower total ventricle-to-body weight (TV/BW) and right ventricle-to-body weight (RV/BW) ratios compared with the other groups (P ≤ 0.05). Additionally, omega-3 supplementation significantly reduced mortality rates in birds reared under both SSD and HSD (P ≤ 0.05). Omega-3 supplementation significantly reduced serum concentrations of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) (P ≤ 0.05). Histological analysis showed that birds reared under HSD with omega-3 supplementation had thinner pulmonary arterial walls compared with those under HSD without omega-3, as well as those reared under SSD regardless of supplementation. The smallest cardiomyocyte diameter was recorded in birds reared under HSD and supplemented with 0.5% omega-3 (P ≤ 0.05). Gene expression analysis revealed that acetyl-CoA carboxylase alpha (ACACA) and Phosphofructokinase-1 (PFK1) were significantly down-regulated in the liver tissue of birds reared under HSD compared with those fed HSD diets supplemented with omega-3 (P ≤ 0.05). Overall, omega-3 supplementation was associated with reduced mortality, lower liver enzyme levels, attenuated histological alterations in the pulmonary artery and RV, and modulation of hepatic genes related to energy metabolism.</p>