Seaweed (SW) is employed as filler to prepare composites on the basis of a polypropylene (PP) matrix in the ratio of 10 : 90, 20 : 80, 30 : 70, 40 : 60, and 50 : 50 (wt % SW : wt % PP) by compounding and injection molding. The tensile, bending and impact properties of the composites were investigated. The 30% SW : 70% PP composite showed the best over-all mechanical performance of the composites prepared. Further improvement of this optimal composite was achieved by incorporating 2% thermoplastic elastomer (TPE) as additive. Interfacial adhesion and bonding between the fibers and PP matrix were investigated by scanning electron microscopy (SEM). Water absorption tests of the different composites were also performed, and addition of TPE was found to lead to a substantial reduction of water uptake.