An alternative strategy was developed to prepare intramolecularly crosslinked soft nanoparticles using the aza-Michael addition reaction of a linear polyester (PCA) containing in-chain reactive triple bonds. The linear precursor was achieved using the conventional polyesterification method in the presence of an acid catalyst. Single-chain polymeric nanoparticles were successfully synthesized by the aza-Michael addition route using a bifunctional amine compound and PCA in a short reaction period (15 min). The resulting polymeric nanoparticles exhibited lower hydrodynamic radii compared to their linear analogues (Rh < 5.55 nm). Crosslinked particles transformed into globular morphologies due to the collapse process as well. Additionally, the achieved nanoparticles were exploited for post-modifications via a thiol-Michael addition reaction.