Plastic pollution poses a significant global environmental challenge. Bioplastics, particularly those derived from sustainable seaweed aquaculture, present a promising opportunity to mitigate this issue. However, the environmental impacts of seaweed bioplastics are still largely unexamined. We conducted a ‘cradle-to-grave’ life cycle assessment (LCA) of a seaweed bioplastic using data from a British extruded bioplastic company. Seaweed was sourced and processed for polymer extraction initially in Indonesia, before being transported to the United Kingdom for final extrusion processing and sale. We used primary input data for the entire supply chain aside from end of life. We found that polymer extraction accounts for the majority of environmental impacts, contributing 74% of total emissions, where the entire footprint amounts to 7.8 kg CO2 eq per kg of extruded pellet. Seaweed farming, including carbon sequestration from biomass burial in sediments below the seaweed farm and excluding temporary biogenic carbon storage, contributed to just 3.6% of overall emissions. We compared impacts of the company bioplastic to polylactide (PLA) plastic which showed PLA had lower impacts, between 10 and 40%, in all impact categories except stratospheric ozone depletion. Nonetheless, we note that several of the positive environmental benefits of the seaweed bioplastic and the negative impacts of PLA for instance from plastic pollution are not captured in existing LCA data. In addition the company's bioplastic pellets are a novel product, while PLA production has benefited from decades of optimisation in large-scale industrial processes. To reduce the impacts across the full life cycle of the company, close collaboration with partners on the seaweed farming and extraction stages will be essential. Overall, seaweed bioplastics are an emerging sector with potential to reduce packaging environmental impacts as it scales up, and more LCAs will be needed to ensure optimized, low-impact processes as production ramps up in the coming years.