Marine ecosystem represents a wide and untapped reservoir of biologically active metabolites. Most of the sponges, seaweeds, crustaceans, fish species and their associated microorganisms have evolved chemical means in order to defend themselves against predation and to survive in the complex marine environment. Such chemical and biological adaptation produced many bioactive substances with beneficial effects on human health, including potential anticancer agents. With further exploration of sea and ocean environment, several marine bioactives have been identified with promising anticancer activity. More than one hundred novel bioactive compounds with antitumor activity and their synthetic derivative substances displayed in vitro cytotoxic property on neoplastic cell lines and are currently gaining great attention for further evaluations and in vivo applications. This review highlights novel marine molecules and compounds which have been able to inhibit different cancer species in the recent years and aims to describe research on new natural antineoplastic agents obtained from sea world.