This study investigated how lignin-used as a natural adhesion promoter in biodegradable, thermoplastic cotton fibre-reinforced composites-influences the composites' mechanical properties. Composites with fibre mass proportions of 40% were produced by compression moulding. Poly(lactic acid) (PLA), a biopolymer, served as matrix. Cotton/PLA composites with and without lignin content were manufactured. As reference samples of bast fibre-reinforced composites, kenaf/PLA composites were produced under the same conditions. The composites were tested for stiffness, tensile strength, elongation at break and impact strength. Fractured surfaces were analysed using scanning electron microscopy (SEM). The results of the composite investigations showed that the addition of lignin has an influence on the cotton/PLA composite characteristics. SEM investigations showed that the adhesion between fibre and matrix could be improved by the addition of lignin. Tensile characteristics like tensile strength and Young's modulus could be improved clearly, while the impact properties were decreased.