The inherent brittleness of poly(lactic acid) (PLA) limits its use. This study demonstrates that blending with poly(butylene succinate-co-adipate) (PBSA) in combination with a chain extender (J-4468) overcomes this limitation. A transition in mechanical behavior is observed, with optimal toughness achieved in PBSA-rich blends. A full series of compatibilized PLA/PBSA blends was prepared and characterized. The results show a substantial improvement in toughness. The PLA50 blend exhibits a fundamental transition to a ductile state, with a notable increase in notched impact strength. The PLA20 blend achieves an optimal balance of properties, combining improved notched impact strength with elongation at break of 225.7%. Both blends show a several-fold increase in tear strength. Although tensile strength and modulus decrease, the heat deflection temperature remains stable near 53 °C. Rheological analysis confirms tunable processability, and morphological analysis confirms the transition from brittle to ductile fracture behavior. The integrated dataset provides a clear structure-property relationship, establishing a formulation strategy for high-toughness, biodegradable PLA-based materials. The observed property combinations suggest potential for flexible packaging applications, pending further validation of barriers and end-use performance.