Functionalized poly(butylene succinate) (PBS) samples were prepared by a post-polymerization method based on the coupling reaction between TEMPO derivatives bearing different functionalities and PBS macroradicals generated by H-abstraction using a peroxide. 4-Benzoyloxy-2,2,6,6- tetramethylpiperidine-1-oxyl (BzO-TEMPO) and 4-(1-naphthoate)-2,2,6,6-tetramethylpiperidine-1-oxyl (NfO-TEMPO), a pro-fluorescent nitroxide, were successfully grafted on PBS, as revealed by MALDI TOF MS and UV-Vis spectroscopy. The functionalization degrees were accurately determined by UV-Vis analysis and confirmed by 1H-NMR spectroscopy. The grafting site was identified by combining theoretical calculations with experimental evidence. This evidence was collected by both EPR analysis of a functionalized sample subjected to controlled heating in the EPR cavity, and by 1H-NMR spectroscopy. Our functionalization method, which was also tested for poly(lactic acid) (PLA), preserves the original polymer structure. This avoids the crosslinking-branching side reaction, which generally affects the free radical treatment of biodegradable aliphatic polyesters. In addition, using a pro-fluorescent nitroxide to form functionalized samples is a significant step towards unambiguously demonstrating the radical grafting on these types of polymer. It also proves that well-defined fluorescently labeled biodegradable polyesters can be tailored.