The transition from a fossil-based to a biobased chemical sector requires developing technologies to produce biobased chemicals that can replace the current market needs already supplied by fossil-based chemicals. Terephthalic acid is an important monomer for producing polyethylene terephthalate typically used to manufacture plastic bottles and textiles. Alternatively to the fossil-based route (naphtha cracking), terephthalic acid can be produced using biomass as feedstock through biochemical and thermochemical routes. The biochemical route involves the chemical and enzymatic conversion of biomass to sugars, while the thermochemical involves the thermal conversion to bio-oil. This study compares the environmental performance of both routes for terephthalic acid production using Miscanthus as feedstock. Miscanthus is a potential crop for biofuels and biochemicals due to its highly efficient use of water and resources (nitrogen, phosphorus). A life cycle assessment framework is used to quantify the environmental impact of both conversion routes using a cradle-to-gate approach. The influence of the inventory data quality and the contribution to the environmental impact is evaluated using uncertainty and hotspot analyses. The thermochemical route showed better environmental performance (higher than 50%) in most selected impact categories than the biochemical route. Energy requirements and enzyme production were the main contributors to the performance of the thermochemical and biochemical routes, respectively.