Fucoidan, a sulfated polysaccharide purified from brown algae including Fucus vesiculosus and Laminaria japonica, has a variety of biological activities, including antioxidant and antitumor activities. Here, we investigated the radioprotective effects of fucoidan on human monoblastic leukemia cell line U937. Further, animal tests were carried out using Balb/c mice in order to determine the radiation-induced changes in the counts of blood cells, including thrombocytes, erythrocytes, leukocytes and hematocrit. Cell viability was assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, wherein fucoidan (1, 10, and $100{mu}g/mL$) was observed to improve recovery from damage caused by 8-Gy radiation in a dose dependent manner. The viability of U937 cells pre-treated with fucoidan also increased in a dose dependent manner. Furthermore, fucoidan at 100 mg/kg was found to protect against changes in the counts of blood cells as follows: on day 28 after irradiation, the thrombocyte count in the irradiated controls decreased to 45% compared with the non-irradiated controls, while that in the fucoidan-treated group was 60%. The hematocrit in the fucoidan-treated group recovered to 75% on day 28, while that in the irradiated control was 68%. The erythrocyte count in the irradiated controls consistently ranged from 64% to 67% throughout the experiment, but that in the fucoidan-treated group increased gradually, ranging from 75% to 80%. The mean number of survival days and 50-day actuarial survival rate increased dose dependently in the fucoidan-treated group. The mean number of survival days and the 50-day actuarial survival rate in this group was 16, 21, and 29 days and 12%, 20%, and 30% at fucoidan doses of 1, 10, and 100 mg/kg. The values of these parameters in the control group were 9 days and 0%, although the difference between the test and control groups was not statistically significant. Our results may prove valuable in the field of radioprotection.