This study evaluated some physical and mechanical properties of injection-moulded polypropylene (PP) composites reinforced with various mixtures of the wood flour (WF) and the olive mill sludge (OMS). Water resistance of the composites improved by the increasing OMS flour content. However, the flexural properties of the composites decreased with increasing OMS flour content. The addition of a coupling agent improved the compatibility between the lignocellulosic material and PP through esterification and thus reduced the water absorption and improved the stability and mechanical properties. The findings obtained in the study showed that the OMS was capable of serving as new reinforcing filler in the manufacturing of thermoplastic polymer composites.