Understanding the dynamic behavior of molecular self-assemblies with higher-dimensional structures remains a key challenge to obtaining well-controlled and monodispersed structures. Nonetheless, there exist few systems capable of realizing the mechanism of supramolecular polymerization at higher dimensions. Herein, we report the unique self-assembling behavior of polyion complexes (PICs) consisting of poly(ethylene glycol)-polyelectrolyte block copolymer as an example of two-dimensional supramolecular living polymerization. Monodispersed and submicrometer unilamellar PIC vesicles (nano-PICsomes) displayed time-dependent growth while maintaining a narrow size distribution and a unilamellar structure. Detailed analysis of the system revealed that vesicle growth proceeded through the consumption of unit PICs (uPICs) composed of a single polycation/polyanion pair and was able to restart upon the further addition of isolated uPICs. Interestingly, the resulting vesicles underwent dissociation into uPICs in response to mechanical stress. These results clearly frame the growth as a two-dimensional supramolecular living polymerization of uPICs.