The effect of adding poly(ethylene co-acrylic acid) (PE-co-AA) in rubber seed shell flour (RSSF)-filled polypropylene (PP) composites on processing, mechanical, water absorption, and morphological properties was studied. The addition of PE-co-AA increased the processing torque in PP/RSSF composites. Furthermore, mechanical properties such as tensile strength, flexural strength impact strength, Young's modulus, and flexural modulus show significant improvement compared to uncompatibilized composites, while elongation at break is reduced. The scanning electron microscopy (SEM) of tensile fracture specimens revealed that the dispersion of RSSF filler in the matrix improved with PEco-AA, with fewer agglomerations. Water absorption ability of the compatibilized composites was reduced due to the improved interfacial bonding, which limits the amount of water molecules to be absorbed. PEco-AA acted as a compatibilizer in PP/RSSF composites. Improvement in the properties of the composites was contributed by homogenization of RSSF particles in the PP matrix and reduction of voids and cracks due to improved interfacial adhesion and bonding between matrix and filler.