Geo-engineering and environmental modification techniques are increasingly being proposed as climate change mitigation strategies. Ocean fertilisation has been promoted as a simple solution to the problem of increasing atmospheric CO2 levels. However, neither its environmental safety nor its efficacy has been adequately assessed. This article examines the legality of ocean fertilisation under the law of the sea and concludes that it is subject to regulation under the London Convention and London Protocol as its potential for harm is contrary to the aims of these agreements. Hence, the sale of carbon offsets to fund ocean fertilisation activities should be prohibited unless and until an adequate risk assessment based on independent peer-reviewed science has established that the benefits outweigh the potential for harm, and appropriate regulation is in place to ensure that real, measurable, long-term CO2 sequestration can be independently verified. The initial uncertainties surrounding the appropriate regulatory regime for ocean fertilisation highlight the need for a comprehensive global regime for the prior assessment and on-going monitoring of existing, new and emerging high seas activities and uses to ensure they do not have adverse impacts on marine biodiversity and the marine environment in areas beyond national jurisdiction.