<p>Cancer remains a leading cause of mortality worldwide, with prostate cancer and glioblastoma being particularly challenging to treat due to dysregulated apoptotic and survival pathways. Natural marine products, such as extracts from the diatom <i>Chaetoceros socialis</i>, may offer promising in vitro cytotoxic and pro-apoptotic activities that justify further mechanistic and in vivo investigation towards therapeutic development. This study investigates the anticancer effects of ethanolic extract of <i>Chaetoceros socialis</i> on human prostate cancer (LNCap) and glioblastoma (U-87 MG) cell lines by targeting key apoptotic and survival signaling pathways. Cell viability was assessed using MTT assay following treatment with varying concentrations of the extract. Apoptosis induction was evaluated by Annexin V-FITC/PI staining and flow cytometry. Gene expression levels of apoptosis and survival-related markers, including <i>CASP3, CASP8, CASP9, BAX, BCL2, AKT, PTEN, mTOR, FAS, P53,</i> and <i>P21,</i> were quantified via real-time PCR. Normal HUVEC cells were used to evaluate extract selectivity. <i>Chaetoceros socialis</i> extract significantly reduced viability of LNCap and U-87 MG cells in a dose-dependent manner, with minimal toxicity to normal HUVECs. Apoptosis assays revealed increased early and late apoptotic cell populations in treated cancer cells. Gene expression analysis demonstrated upregulation of pro-apoptotic genes (<i>BAX, CASP3, CASP8, CASP9</i>) and tumor suppressors (<i>PTEN, P53, P21</i>), along with downregulation of survival-promoting genes (<i>AKT, mTOR, BCL2</i>). The ethanolic extract of <i>Chaetoceros socialis</i> exerts selective cytotoxic and pro-apoptotic effects on prostate (LNCap) and glioblastoma (U-87 MG) cell lines in vitro by modulating apoptotic and survival pathways. These results indicate an in vitro anticancer potential that requires further protein-level mechanistic validation and in vivo studies to confirm therapeutic relevance.</p>