Natural products have played a key role in cancer drug discovery, as well in other therapeutic fields. In the past decades, marine organisms have proven to be a primary source of new potentially bioactive natural products for drug discovery. By reviewing the literature describing marine organisms and isolated metabolites, we can notice a large increase in the number of studies today compared to the end of the 20th century. The number of structures isolated each year has almost doubled over the past 20 years. Because of their topicality, we have focused on natural bioactive steroids isolated from marine organisms. In the chapter ‘Bioactive Steroids from Marine Organisms’, an overview of the new steroid compounds isolated from marine sponges, macroalgae and cucumbers, described in the relevant literature in the period from 2011 to 2020, is given. To provide a comprehensive introduction in the field of marine bioactive steroids, we highlighted typical molecules grouped according to their structural characteristics with additional reference to their biological activity. The structures of the new compounds, their natural origin (species of the organism) and their rich biological activities are presented and described in detail. In addition, biological tests performed on known compounds during this time period are also described. Some of the compounds possess multiple activities and have been tested only in a limited number of biological assays, which means that the full potential and significance of these compounds may only be discovered in the future.