The red seaweed Jania rubens (J. rubens) is prevalent along the Lebanese coast and has drawn attention for its notable antineoplastic properties. Our previous data showed that its dichloromethane–methanol (DM) extract possesses antioxidant, cytotoxic, and anti-migratory effects on colon cancer cells. In the present study, a GC-MS analysis of DM extract identified a diverse profile of bioactive compounds, including flavonoids and pyrazole derivatives with antioxidant and anticancer activities. In vitro assays demonstrated that the DM extract exerts significant cytotoxic activity against various cancer cell lines, including colon, breast, and cervical types. Further investigation into the underlying molecular mechanisms revealed that the extract induces G2/M cell cycle arrest and reduces the expression of EMT (epithelial–mesenchymal transition) markers, N-cadherin and Twist. In addition, the extract showed anti-metastatic properties through its ability to decrease MMP-2 and MMP-9 activity. Mechanistically, DM caused a substantial reduction in Ten-Eleven Translocation (TET) enzymes TET-1, TET-2, and TET-3, which are essential DNA demethylation regulators, thus decreasing their enzymatic product 5-hydroxymethylcytosine (5-hmC). Interestingly, despite a significant increase in intracellular ROS (reactive oxygen species), suggesting a contribution to cytotoxicity, no substantial change in the biogenesis of promyelocytic leukemia nuclear bodies (PML-NBs) was detected. These findings demonstrate that J. rubens DM extract contains bioactive compounds with multiple anticancer effects, thus making it a promising candidate for developing new therapeutic agents.