In cancer treatment, increase in drug resistance and decrease in new chemotherapeutic drugs have become a pressing problem. Hence, searching for novel anticancer agents with less toxicity and high sensitivity is expanding gradually. Many preclinical and clinical studies indicate that natural antioxidants can help combating carcinogenicity and reduce the adverse effects on cancer therapy, when used alone or as adjuvant in chemotherapy. Consequently, marine algae pave the way for exploring more potential antioxidant compounds which have pharmaceutical importance. Algal terpenoids comprise a large group of bioactive compounds that have excellent antioxidative property and can be used as source of antioxidant in cancer therapy. This chapter summarizes the potential role of terpenoids from algal sources in inhibiting cancer cells, blocking cell cycle, hindering angiogenesis and metastasis as well as in inducing apoptosis.