The present study aimed to investigate the biological effect of Forssk (Achillea fragrantissima) aqueous extract on the immune system of experimental rats. Thirty-five adult male rats (Sprague Dawley strain) were randomly divided into five equal groups (n=7). Group 1 was the control negative group, whereas the other 4 groups were injected with cisplatin (3.5 mg/kg) once every 3 days for 2 consecutive weeks. Group 2 was kept as the immunotoxic control group (+ve group). Group 3 was orally administered 1 ml/day of Forssk (Achillea fragrantissima) aqueous extract at a concentration of 5%, while Group 4 received 1 ml/day of Forssk (Achillea fragrantissima) aqueous extract at a concentration of 7.5%. Group 5 was orally administered 1 ml/day of Forssk (Achillea fragrantissima) aqueous extract at a concentration of 10% for 8 weeks. Results showed that intraperitoneal injection of cisplatin in rats caused a significant (P<0.05) reduction in body weight, spleen weight, serum immunoglobulins (IgG, IgM, and IL-1B), total protein, albumin, globulin, leukocytes, and neutrophils, whereas lymphocytes and monocytes significantly increased in the positive control group compared to the negative control group. On the other hand, oral administration of Forssk (Achillea fragrantissima) aqueous extract attenuated these adverse effects and markedly ameliorated biochemical and functional  alterations in the spleen caused by cisplatin injection. In conclusion, Forssk (Achillea fragrantissima) aqueous extract stimulates the immune system of rats with cisplatin-induced suppressed immunity. This study recommends increasing the consumption of Forssk (Achillea fragrantissima) in the diet, as it may raise the immune response in patients with immune disorders.