In this paper, raw hemp fibres were used to reinforce a commercial biopolyester, namely poly(1,4-butylene adipate-co-terephthalate), to achieve eco-composite materials. To promote the adhesion between the polyester matrix and the fibrous reinforcement, chemical modification of fibres surface with coupling agents was accomplished. Usually, the grafting of coupling molecules is carried out involving high amount of solvents. Here, in a general approach of an environmental sustainability, uncut, combed and slightly stretched hemp fibres were treated with a green process based on a novel soft plasma jet device (developed by Nadir Srl). This technique activates the fibres surface and, at the same time, allows the deposition of reactive monomers as coupling agents. Three different monomers were investigated: methyl methacrylate (MMA), (3-aminopropyl)triethoxysilane (APTES) and (3-glycidyloxypropyl)trimethoxysilane. Unmodified and surface-modified fibres were characterized and compared in terms of thermal behaviour and morphological features. Long-fibre eco-composites were then prepared by different compression moulding procedures and characterized by thermal, morphological and dynamic mechanical analysis. Experimental evidences indicated that the cold plasma deposition/grafting treatment was successfully carried out; the composite with the best mechanical performance resulted that obtained by using APTES as coupling agent.