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This work reports the development and characterization of green composites based on polylactide (PLA) containing fillers and additives obtained from by-products or waste-streams from the linen processing industry. Flaxseed flour (FSF) was first produced by the mechanical milling of golden flaxseeds. The resultant FSF particles were melt-compounded at 30 wt% with PLA in a twin-screw extruder. Two multi-functionalized oils derived from linseed, namely epoxidized linseed oil (ELO) and maleinized linseed oil (MLO), were also incorporated during melt mixing at 2.5 and 5 parts per hundred resin (phr) of composite. The melt-compounded pellets were thereafter shaped into pieces by injection molding and characterized. Results showed that the addition of both multi-functionalized linseed oils successfully increased ductility, toughness, and thermal stability of the green composite pieces whereas water diffusion was reduced. The improvement achieved was related to both a plasticizing effect and, more interestingly, an enhancement of the interfacial adhesion between the biopolymer and the lignocellulosic particles by the reactive vegetable oils. The most optimal performance was attained for the MLO-containing green composite pieces, even at the lowest content, which was ascribed to the higher solubility of MLO with the PLA matrix. Therefore, the present study demonstrates the potential use of by-products or waste from flax (Linum usitatissimum L.) to obtain renewable raw materials of suitable quality to develop green composites with high performance for market applications such as rigid food packaging and food-contact disposable articles in the frame of the Circular Economy and Bioeconomy.