Natural products (NPs) are useful sources of bioactive compounds and play important roles in the development and discovery of new drugs for diverse human diseases. Most natural products originate from terrestrial species, but diverse marine organisms are another source of new agents for cancer therapy. Natural products derived from marine organisms show diverse pharmacological activities via bioactive secondary metabolites. They regulate biological activities, such as cell proliferation, cell viability, induction of ROS production, ER stress, and apoptosis via modulation of cellular mechanisms in many cancers. Many natural products isolated from marine species require further study to elucidate the efficacy of their biological activity and anticancer effects. In this review, we summarize the biological properties and anticancer effects of diverse natural products extracted from marine organisms and their roles in tumor therapy. Cancer is one of the leading causes of death and an economic burden worldwide (1). Cancer involves an abnormal proliferation of cells and tissues, and is affected by various risk factors such as age, diet, genetics, and environmental factors (2, 3). It is also caused by the mutation of several cancer-related genes called tumor suppressors and oncogenes (4). Cancer also presents the ability of metastasis and recurrence, which are difficult to treat (5). Although conventional anticancer therapy includes both chemotherapy to induce apoptosis of target cancer cells and surgical therapy to remove tumors, these methods are limited by issues of therapeutic efficacy, safety, and side effects. Thus, it is necessary to develop novel strategies for cancer therapy, and natural products (NPs) are a promising source for novel drug development because they show diverse biological activities and anticancer effects via inhibition of tumor growth by interacting with several signaling pathways (6, 7). NPs have been a useful source of bioactive compounds and play an important role in the discovery and development of drugs. Most NPs, originated from terrestrial microbes, fungi, and plants, have been discovered to possess many pharmacologically active factors and used to treat several human diseases. Some of these are utilized as clinically valuable drugs for anticancer therapy (8-10). However, marine organisms have recently attracted substantial attention because 70% of the earth is covered by water, representing 95% of biodiversity (11-13). NPs derived from marine organisms have been researched for novel bioactive secondary metabolites due to their diverse pharmacological activities. Marine environments are estimated to harbor more than one million species and one billion different types of marine microbes (14). Today, around 28,000 new compounds isolated from marine species, such as algae, seaweed, sponges, and starfish, have been reported, and this species diversity provides a diverse array of secondary metabolic products (15, 16). In this review, we present NPs derived from marine environments, and summari...