The interest in nanocellulose made from woody biomass has been growing rapidly; however, detailed studies on the environmental performance of nanocellulose have only been reported on a few occasions. To fulfill this gap, the environmental performance of nanofibrillated cellulose fabricated from thermo-groundwood (removal of extractives, lignin and hemicelluloses, TEMPO oxidation and homogenization processes were included) was evaluated by means of a Life Cycle Assessment. The results show that the purification process contributes more than 95% of the impact. It is associated with a relatively high consumption of electrical energy and ancillary chemicals, i.e., cyclohexane and acetone. The global warming potential of 1 kg of nanofibrillated cellulose is as high as 800 kg CO₂ equivalents. Even in the case that in addition to the extractives and the hemicelluloses also lignin is considered as a potentially valuable co-product, and the latter takes over some of the burden, the impact of nanofibrillated cellulose remains relatively high, at around 400 kg CO₂ equiv. per kg of nanocellulose. While the primary energy consumption is around 19.000 MJ per kg of nanofibrillated cellulose, or 10.100 MJ in the case that the lignin is considered as a potentially valuable co-product. The study also had a methodological goal, i.e., the impact indicators were calculated using the three most relevant evaluation methods: ILCD/PEF, CML 2001 and ReCiPe 2016. These three methods show similar results for the impact on global warming and acidification. However, in the case of impacts on some other indicators, significant deviations in the obtained impact scores were observed with respect to the results for the three methods. Taking into account the background data of the methods, ReCiPe 2016 was found to be the most up-to-date method and can currently be considered as the preferable Life Cycle Impact Assessment method.