This study presents a sustainable bioconversion strategy for upcycling biodegradable plastics into polyhydroxyalkanoates (PHAs) using Paracoccus sp. LL1. Alkaline hydrolysis was used to depolymerize three examples of bioplastics: poly(lactic acid) (PLA), poly(butylene succinate) (PBS), and poly(butylene adipate-co-terephthalate) (PBAT). The resultant hydrolysates were evaluated as carbon substrates for microbial PHA production. Under optimal conditions, PLA and PBS exhibited 100% hydrolysis, while PBAT demonstrated a 69% hydrolysis efficiency. PLA hydrolysate produced the highest biomass concentration (7.37 g/L) and PHA production (1.88 g/L), followed by PBS (0.98 g/L) and PBAT (0.27 g/L), according to shake-flask cultivation. During flask-scale cultivation with 50 g/L bioplastic hydrolysate, the maximum PHA content reached 26% (w/w). The polymer was primarily composed of 3-hydroxybutyrate (3HB) and less than 3% 3-hydroxyvalerate (3HV). Batch cultivation in a 5 L bioreactor using PLA hydrolysate further enhanced biomass and PHA production to 11.3 g/L and 3.69 g/L, respectively, corresponding to a PHA content of 32.9% at 96 h. Fourier transform infrared spectroscopy confirmed the characteristic ester carbonyl (CO) and CO stretching vibrations of PHA, while thermogravimetric analysis indicated high thermal stability up to 250 °C. Overall, this study demonstrates the technical feasibility of converting bioplastic hydrolysates into value-added PHAs, offering an integrated approach for biodegradable plastic waste valorization within a circular bioeconomy framework.