Two organofunctional silanes and a copolymer were used to increase the interfacial adhesion in glass fiber polypropylene (PP) reinforced composites. The performance of the coupling agents was investigated by means of mechanical property measurements, scanning electron microscopy (SEM), and dynamic mechanical analysis. The increased adhesion between the glass fibers and PP matrix observed with SEM resulted in an improvement of the mechanical and dynamic mechanical properties of the composites. Coupling achieved with the copolymer poly(propylene-g-maleic anhydride) (PP-g-MA) proved to be the most successful compared with 8-aminopropyltrimethoxysilane and 3-aminopropyltriethoxysilane. The combination of PP-g-MA with the silanes resulted in further property improvements because of the ability of the MA groups to react with the amino groups of the silanes.