Blue Economy is seen as an essential contributor to a sustainable development, and it is an important part of the EU Green Deal. Seaweed plays a key role in the Blue Economy as a source of food, feed, and feedstock for biorefineries. Today, the largest part of global seaweed production is based in Asia, but there is also a growing interest in seaweed production in Europe. However, more knowledge on the environmental impacts is needed to ensure sustainable growth of the sector. Seaweed can be used in biorefineries to produce a variety of products for food and non-food applications. The aim of this paper was to perform a life cycle assessment (LCA) of a seaweed value-chain, including seaweed cultivation and production into sodium alginate, biodegradable materials, biogas, and fertilizer in a biorefinery setting. The LCA included 19 environmental impact categories but focused on climate change. The seaweed Saccharina latissima was cultivated and processed in Ireland. Sodium alginate was then extracted by means of ultrasound-assisted extraction, a novel extraction technology. Cellulosic residues produced after the extraction were used for the production of films used as a packaging material. Residues that remain after the production of the films were sent to anaerobic digestion to achieve a no-waste concept. For seaweed cultivation, fuel use and drying of seaweed biomass were the main environmental hot spots; and for the alginate extraction process, the yield and purification after extraction were the main hot spots. Overall, the results of this paper showed that the seaweed-based biorefinery has the potential to be sustainable, but several improvements are necessary before it is competitive with land-based systems. © 2022 The Authors