Background: Hepatocellular carcinoma is one of the most common types of malignancy and causes significant morbidity and mortality worldwide. Gracilaria foliifera (Forssk.) Børgesen, a brown marine alga, is shown to have growth inhibitory potential against various cancer cell lines other than human hepatoma HepG2 cells. Objective: To investigate the cytotoxic potentials of G. foliifera in HepG2 cells. Materials and Methods: HepG2 cells were fed with culture medium supplemented with different concentrations of ethanolic extract of G. foliifera (20, 40, and 80 μg/mL). After 24 h of treatment, the cytotoxicity was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Induction of early apoptosis was investigated by annexin V-fluorescein isothiocyanate immunofluorescence. Induction of late apoptosis and necroptosis was investigated by annexin V and propidium iodide (PI) staining. Nuclear chromatin condensation was evaluated by Hoechst staining. p53 protein expression was analyzed using Western blotting. Results: G. foliifera treatment in HepG2 cells caused a significant cytotoxic effect. Phosphatidylserine translocation confirms the induction of early apoptosis. Analysis of late apoptosis using annexin V/PI staining showed that the percentage of apoptotic cells was increased in a concentration-dependent manner. Hoechst nuclear staining further confirms the nuclear chromatin condensation. G. foliifera treatment also induced the tumor suppressor p53 protein expressions. Conclusion: The present study demonstrated that G. foliifera induced apoptosis in HepG2 cells through activation of p53.