Compounds obtained from natural resources have become important candidates in the discovery and development of drugs. Over the past few decades, marine resources have gradually attracted a large amount of attention from researchers, and many compounds with varying structures and activities have been derived from various marine resources. Since the ocean experiences more variable temperatures, lower oxygen levels and less light than the terrestrial environment, marine resources offer a substantial number of organisms that are not available or rarely detected on land. Researchers expect that they can discover agents from marine organisms that exhibit good activity for diseases. Of the marine compounds acquired, the majority have shown to have good antitumor activity, and range in phases from clinical trials up to being used in the clinic. These natural products operate through different pathways and have unique antitumor effects that include inducing cell apoptosis, inhibiting cell proliferation and influencing cell migration. A variety of drugs have been used for cancer treatment during the last few years, but it has not been possible to greatly extend the survival time of advanced cancer patients. In addition, researchers are actively looking for compounds to overcome problems that include a lack of effective drugs to control the transfer of cancer cells and the development of drug-resistant microbes. Here, we summarize the marine natural products that have been obtained over the past few years, and analyze their antitumor effects and mechanisms. Our research will provide a significant basis for anticancer drug screening and development.