Diazinon (DZN) is a widely used pesticide that can affect the vital organs of non-target aquatic animals—mainly fish. This study evaluated the acute toxicity (LC₅₀) of six DZN concentrations (0.5, 0.75, 1.5, 2, 2.5, and 3 mg·L⁻¹) and, based on its evaluation after 24 h, 48 h, 72 h, and 96 h, two sublethal concentrations for chronic toxicity testing (0.01 and 0.05 mg·L⁻¹) for 21 days of exposure to DZN on grass carp fingerlings (Ctenopharyngodon idella Valenciennes, 1844). The median lethal concentrations of DZN at 24, 48, 72, and 96 h were 1.83, 1.57, 1.35, and 1.12 mg·L⁻¹, respectively. Next, histological observations after 96 h LC₅₀ showed oedema of the primary lamellae of the gills at low pesticide concentrations (0.5 to 1 mg·L⁻¹) and extensive necrosis of primary lamellae of the gills at higher concentrations (1.5 to 3 mg·L⁻¹). Moreover, cytoplasmic vacuolation and extensive necrosis were observed in liver tissue. Increased skin mucus, unbalanced swimming on the water surface, and increased gill opercula movements were noted during chronic exposure. Haematological parameters such as haematocrit, red blood cell count, white blood cell count, haemoglobin, and mean corpuscular volume were significantly reduced after 21 days of exposure to 0.05 mg·L⁻¹ of DNZ (p < 0.05). The present study shows that DZN has various toxic effects on grass carp, which may pose a potential risk for other fish species.