Marine floras, constituting over 90% of the oceanic biomass, are rich sources of potent chemicals predominantly belonging to polyphenols and sulphated polysaccharides. They act as potential sources of drugs with various pharmacological properties (such as antioxidant, anti-inflammatory, antibiotic, immunostimulatory, and anticancer). Although human history depicts the use of marine floras as medicine and nutrient supplement, the search for novel pharmaceutical compounds has mostly been limited to terrestrial floras. Marine floras are taxonomically diverse, largely productive, biologically active, and chemically unique, thereby offering great scope for discovery of new anticancer drugs. The mode of action of marine flora products is through activation of macrophages, induction of apoptosis, and prevention of oxidative damage of DNA, thereby controlling carcinogenesis. This chapter focuses on the immunomodulatory and therapeutic properties of marine flora-derived products in the context of increasing cancer incidences and demand for cheaper, safer, and potent anticancer drugs. It highlights the excellent therapeutic potential of the known marine flora-derived anticancer compounds, thereby encouraging researchers to actively participate in the marine natural product drug discovery for treatment and prevention of cancer.