This study aimed at investigating the reinforcement effect of milkweed (MW) floss, a smooth and homogeneous natural fiber with a wide hollow lumen, on bio-based polymer composites. First, MW floss was thoroughly characterized in terms of morphology, surface roughness, and tensile and thermal resistance. Then, MW floss was compared to flax fibers, one of the most widely used natural fibers in the composite industry. Subsequently, bio-based composites made of polylactic acid (PLA) and 1 wt% MW floss were produced by injection molding and compared to composites reinforced with 1 wt% of flax fibers. Finally, thermal behavior, mechanical properties, and impact resistance of composites were determined. Results showed that MW floss, with respect to flax fibers, exhibits lower tensile modulus, ultimate tensile strength, surface roughness as well as a shorter critical length. Nonetheless, and despite the lower intrinsic properties of MW floss, UTS and impact resistance of MW/PLA composites were found to be 60% and 15% higher than those of Flax/PLA composites, respectively. In addition, micrographs of MW/PLA interface revealed a lack of adhesion in MW/PLA, which should be overcome by surface treatment in upcoming work.