This chapter critically assesses marine geoengineering as a novel and controversial use of the marine environment from the perspective of the law of the sea. It begins with an assessment of the first and thus far, only, regulatory framework specifically designed to manage a subset of geoengineering activities, ocean fertilization: the 2013 amendments to the 1996 Protocol to the 1972 London (Dumping) Convention. The chapter goes onto explore the extent to which the new framework developed in these amendments provides an appropriate and robust response to the environmental and ethical challenges posed by ocean fertilization activities. In light of the fact that the 2013 amendments are not yet in force and will, once they enter into force, bind only a relatively few states in respect of a very narrow range of geoengineering activities, the chapter then proceeds to examine the broader rights and responsibilities incumbent on states under the 1982 United Nations Convention on the Law of the Sea (LOSC) and the dumping regime with respect to marine geoengineering activities more generally. In particular, the chapter explores the extent to which global obligations to protect the marine environment and to control scientific research provide appropriate tools for managing the risks posed by geoengineering. Finally, the chapter concludes with some thoughts on the risks of developing a regulatory regime for geoengineering under the law of the sea in isolation from a broader ethical debate over the relationship between geoengineering, emissions reductions and climate change mitigation more generally.