A novel polymeric flame retardant, PBPP2, for polylactide (PLA) was synthesized via a two-step polymerization process. The flame retardancy of neat PLA and flame-retarded PLA films prepared via solution casting method with different PBPP2 ratios was then investigated. When only the 3wt% PBPP2 was added to PLA, the limiting oxygen index was increased from 23 to 30%. Additionally, the required V-0 rating was achieved compared to neat PLA. It was also found that PBPP2 increases the maximum thermal decomposition temperature of PLA, indicating that 3wt% of PBPP2 is sufficient to improve flame retardancy and thermal stability on PLA, simultaneously. From both the decrease in T-m and the increase in elongation for PLA with the 3wt% addition of PBPP2, PBPP2 could be considered to also function as a plasticizer on PLA.