Background: Spirulina platensis, an edible cyanobacterium, is considered as a valuable and natural resource of novel anticancer agents. This study aimed to investigate the anticancer potential of major bioactive metabolites from Spirulina platensis on hepatocellular carcinoma cells. The total phenolic and alkaloid content of S. platensis were determined using spectrophotometric procedures and thin-layer chromatography. Cellular viability of HepG2 cancer cells and normal fibroblasts was evaluated using MTT assay after 24 h treatment with 0.02–2 mg/ml of alkaloids, phenolic compounds, aqueous, and methanol extracts from Spirulina platensis. Results: Total phenolic and total alkaloid compounds were 150.5 ± 1.18 mg gallic acid equivalents/mg extract and 11.4 ± 0.05 mg atropine equivalents/mg extract, respectively. All tested extracts and compounds demonstrated the inhibitory effect on the viability of HepG2 cells in a dose-dependent manner without cytotoxicity on normal cells. The most potent anticancer activity was induced by alkaloids (2 ± 0.001 mg/ml) with 80% reduction in cell viability and an IC50 of 0.53 ± 0.08 mg/ml. IC50 values of the aqueous extract, the methanolic extract, and phenolic compounds were 1.7 ± 0.14, 1.28 ± 0.22, and 0.86 ± 0.14 mg/ml, respectively. Conclusions: This is the first report to demonstrate anticancer effects of alkaloids and phenolic compounds of Spirulina platensis in relation to liver cancer.