Residual softwood sawdust was pretreated by a steam-explosion technique. It was used as a natural filler in polypropylene (PP)-based composites. Dynamic mechanical analysis and tensile properties of these materials were studied. The influence of filler loading, steam-explosion severity, and coating the fiber with a functionalized compatibilizer, such as maleic anhydryde polypropylene (MAPP), on the mechanical behavior of the composite was evaluated. The results were analyzed in relation with scanning electron microscopy observations, and surface energy (dispersive and polar components) and apparent specific area measurements. Experimental data indicate a better compatibility between MAPP-coated fiber and PP with respect to the untreated one. The coating treatment of the softwood fiber was found to promote interfacial adhesion between both components, and to enhance the tensile properties of the resulting composite. This reinforcing effect was well predicted from theoretical calculations based on a mean field approach (Halpin-Kardos model). The steam-explosion pretreatment severity increased the surface energy and apparent specific surface, and resulted in a loss of the fiber entirety. The sorption behavior of these composite materials was also performed. It was found that the composites absorb more water, as the filler content is higher. MAPP coating provided protection from water uptake in the interphase region.