Ultrasonified extracts from seawater-based cultures of the microalga Spiyulina platensis were obtained using water and ethanol at 60 and 100$^{circ}C$. The yield of the aqueous fraction of S. platensis extracted using ultrasonification was about 33.46%. The cytotoxicity against HEK293 and inhibition ratios of the cancer cell lines A549, AGS, MCF7, and Hep3B were measured using the sulforhodamine-B (SRB) assay. The cytotoxicity of all extracts at 1.0 mg/mL was below 26%. The cytotoxicity of the ultrasonified extracts from the seawater-based culture of the microalga Spirulina platensis was about 4% less than that of Spirulina platensis without ultrasonification. The inhibition ratio of cancer cell growth was approximately 80% for 1.0 mg/mL extracts. The inhibitory effect on cancer cell growth was greater for seawater containing ultrasonified Spirulina platensis extracts than for extracts without ultrasonification. The differentiation ratio of HL-60 cells was 160.9%. Densitometric analysis of Bcl-2 revealed that the ultrasonified extracts had greater anticancer activity than the extracts without ultrasonification.