This work explores the use of supercritical carbon dioxide (sc-CO2) conditions as an innovative and environmentally friendly treatment of plant fibres to optimize their performance for integration into composite materials. This study evaluates, in particular, the influence of this treatment on the mechanical, thermal, hygroscopic properties and biochemical features of industrial hemp bast fibres. Two distinct settings were tested by tuning time, temperature and pressure parameters to assess the influence of the severity of the treatment on the fibre quality. Results show that sc-CO2 treatment induces an increase in the fibre fineness and a decrease in their moisture sensitivity while maintaining their initial resistance to temperature. These changes are consistent with the measured decrease in the relative content of hemicelluloses. A significant decrease in the tensile rigidity and strength is also observed as a function of the severity of sc-CO2 treatment, counterbalancing a little bit the benefits retained on the other properties.