Bio-based plasticizers derived from renewable resources represent a sustainable replacement for petrochemical-based plasticizers. Vegetable oils are widely available, non-toxic and biodegradable, resistant to evaporation, mostly colorless and stable to light and heat, and are a suitable alternative for phthalate plasticizers. Plasticized poly(lactic acid) (PLA) materials containing 5 wt%, 10 wt%, 15 wt% and 20 wt% natural castor oil (R) were prepared by melt blending to improve the ductility of PLA. Three castor oil adducts with maleic anhydride (MA), methyl nadic anhydride (methyl-5-norbornene-2,3-dicarboxylic anhydride) (NA) and hexahydro-4-methylphthalic anhydride (HA), previously synthesized, were incorporated in a concentration of 15 wt% each in PLA and compared with PLA plasticized with natural R. The physico-chemical properties of PLA/R blends were investigated by means of processability, chemical structure, surface wettability, mechanical, rheological and thermal characteristics. The addition of natural and modified R significantly improved the melt processing by decreasing the melt viscosity by ~95%, increased the surface hydrophobicity, enhanced the flexibility by ~14 times in the case of PLA/20R blend and ~11 times in the case of PLA/15R-MA blend as compared with neat PLA. The TG/DTG results showed that the natural R used up to 20 wt% could significantly improve the thermal stability of PLA, similar to the maleic anhydride-modified R. Based on the obtained results, up to 20 wt% natural R and 15 wt% MA-, HA- or NA-modified R might be used as environmentally friendly plasticizers that can improve the overall properties of PLA, depending on the intended food packaging applications.