Poly(lactic acid) (PLA)/soy protein concentrate (SPC) composites were prepared using a twin-screw extruder. Extra amounts of water and glycerol were added to SPC prior to compounding with PLA to ensure that SPC behaved like a thermoplastic during subsequent mixing. Free radical grafting of maleic anhydride (MA) onto PLA was performed by reactive extrusion, and PLA-g-MA was used as a compatibilizer in the composites. The effects of compatibilizer concentration and degree of functionality on the tensile, morphological, and thermal properties of PLA/SPC composites were studied. The tensile strength of the composites containing 4 phr compatibilizer increased by 19% compared to that of the uncompatibilized one. Also, the presence of compatibilizer resulted in finer domain sizes of SPC and a lower damping peak, both of which suggested that good interfacial adhesion between the two phases was achieved. Differential scanning calorimetry analysis indicated that SPC induced and accelerated cold crystallization of PLA.