Sodium nitrite (NaNO₂) in food preservation causes oxidative stress in the body. Antioxidants are widely used to ward off free radicals caused by oxidative stress. This study aimed to determine the potential of Eucheuma cottonii seaweed extract as a natural antioxidant to improve blood profiles and kidney damage in male albino rats induced by NaNO₂. This research used a completely randomized design. Thirty animals were divided into five groups, namely negative control, positive control, and three treatment doses of E. cottonii extract. NaNO₂ was dissolved in distilled water, and E. cottonii extract was dissolved in 0.5% CMC-Na solution. The research was conducted for 45 days. The treatment group was given NaNO₂ for 35 days; on days 16 to 45, E. cottonii extract was given at doses of 150, 300, and 450 mg/kg BW. The positive control was given NaNO₂ for 35 days, then given the extract solvent. The negative controls were given the solvent only. At the end of the treatment, the rats were sacrificed to collect blood and kidney organs. The results showed that the extract at doses of 150 and 300 mg/kg BW increased the number of hemoglobin and erythrocytes. The dose of 150 mg/kg BW reduced various kidney damage, such as inflammatory cell infiltration, congestion, hemorrhage, fatty degeneration, necrosis, glomerular abnormalities, and protein deposits in the lumen of the renal tubules. The highest dose of 450 mg/kg BW reduced the number of leukocytes.