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Agricultural expansion to meet humanity’s growing needs for food and materials is a leading driver of land-use change and threatens to exacerbate ongoing crises of climate change and biodiversity loss. Seaweed biomass, farmed in the ocean as one facet of the rapidly growing ‘Blue Economy’, could help to mitigate these problems by providing a suitable, even advantageous, substitute for food, animal feed, and biofuels altogether, which could significantly displace demand for terrestrially-produced crops. In addition, recent research has demonstrated that the production of ruminant livestock can be drastically improved by supplementing their feed with the red seaweed Asparagopsis spp. Here we develop a range of scenarios to explore how increasing seaweed utilization may affect land-use change and carbon emissions, and estimate where corresponding sea-use change would occur. For each scenario, we i) use IIASA’s GLOBIOM (Global Biosphere Management Model) to provide a detailed estimation of the terrestrial benefits, and ii) map the geographic potential of 35 commercially important seaweed species and use a spatial optimization algorithm to identify where and how much each could be grown to meet the scenario. Our results show that ca. 349 million hectares of global ocean could support seaweed farms and that cultivating Asparagopsis spp for ruminant feed could mitigate up to 2 Gt CO2e and provides the highest marginal gains for land use. We also find that substituting human diets at a rate of 10% globally would spare up to 100 Mha of natural lands. These findings suggest that several global challenges could be simultaneously addressed by expanding the production of seaweed, however further work is needed to ensure that these farms will be environmentally, technically, and economically viable.