This study investigates hydrothermal carbonisation (HTC) of pulp-and-paper mill sludge to produce hydrochar and enable industrial symbiosis with steelmaking in Sweden. The assessment combines life cycle assessment (LCA) and techno-economic analysis (TEA) to evaluate environmental impacts, production costs, and national-scale implementation potential. The results show that HTC-derived hydrochar reduces the climate impact compared to use of fossil coal in electric arc furnace (EAF) steelmaking, particularly in mill-integrated scenarios that exploit onsite energy and wastewater treatment synergies. Economic viability, however, hinges on CO₂ pricing and hydrochar quality. Without a significant cost penalty for fossil CO2 emissions, hydrochar can be costlier than its fossil counterpart due to lower carbon content. Several pulp mills or mill clusters, in mid- and southern Sweden could support industrial symbiosis arrangements with local steel industries. The nationwide hydrochar production potential, estimated at less than 15% of the projected future carbon demand in Swedish EAFs, indicates a limited role, amplified by substantial regional variations in both supply and demand. Overall, the findings highlight that the suggested industrial symbiosis concept can help mitigate pulp and paper industry waste issues while contributing to reduced fossil coal dependence in steelmaking, forming part of a defossilisation strategy. However, strategic policy frameworks, multi-feedstock approaches, and further optimisation of hydrochar quality will be crucial for realising large-scale adoption.