Introduction: Prostate cancer is one of the most common cancers among men, with an increasing incidence and mortality rate. In the present study, cytotoxic and pro-apoptotic effects of spirulina platensis extract on PC-3 prostate adenocarcinoma cell line were investigated. Methods: In the present experimental study, the PC-3 prostatic cancer cells were treated in four experimental with 400, 200, 100 and 50 μ, g / ml extract of spirulina and incubated at 24 and 48 hours. Cytotoxicity was analyzed by MTS kit (3-(4, 5-Dimethylthiazol-2-yl)-5-(3-Carboxymethoxyphenyl)-2-(4Sulfophenyl)-2H-Tetrazolium, Inner Salt) and apoptosis was analyzed by flow-cytometry using an Annexin V-FITC/PI kit according to the manufacturer protocol in both times. Statistical analysis was accomplished by ANOVA and Duncan tests using FlowJo and SPSS 16 software. Results: In the experimental groups treated with extract of spirulina, the viability of the cells showed a decrease compared to control group, while this decrease was more noticeable in the experimental group of 100 μ, g / ml at both incubation times (P<0. 0071). Increased incidence of apoptosis was significantly higher in the experimental groups than the control group. However, this increase was significantly higher than the control group at concentrations of 200 μ, g / ml in 24h incubation time (Ƥ, < 0. 0331) and 100 μ, g / ml of 48h incubation time (Ƥ,< 0. 0502). Conclusion: Extract of Spirulina at specific concentrations reduced cell growth and induced apoptosis in PC-3 prostatic cancer cells. Evidence suggests that spirulina can be used as an anticancer drug for the treatment of prostate cancer.