The marine environment is facing serious changes. This requires scientific understanding of the ocean’s responses to pressures and management actions to provide the foundation for sustainable development. At the same time, the production of knowledge for a more sustainable world is undergoing rapid change with the uptake of citizen science as means of opening up science. While there is increasing interest in marine citizen science, it is also often conceived as lagging behind citizen science in other areas, such as biodiversity related citizen science on land. This paper analyses empirical evidence collected >1260 past and present marine citizen science initiatives, substantiating the pervasiveness of marine citizen science around the globe. In doing so, it provides the basis for a cumulative inventory of MArine Citizen Science Initiatives (MARCSI) that can inform and shape both the growing community of marine citizen science practitioners and the science of citizen science.