We herein present sequence-controlled polymerization-induced self-assembly (PISA) via photoswitchable reversible addition-fragmentation chain transfer (RAFT) copolymerization of oppositely-charged monomers using polyethylene glycol chain transfer agent in water at 25 °C. Thorough block copolymerization leads to a polymerization-induced electrostatic self-assembly named ABC-mode polymerization-induced electrostatic self-assembly (PIESA), by which PEGylated (PEG, polyethylene glycol) polyion complex (PIC) spheres, lamellae, and vesicles are achieved. We demonstrate the inherent spontaneous zwitterionic alternating copolymerization nature, which leads to the charge-dictated alternating or gradient zwitterionic sequence. As such, we developed sequence-controlled synthesis of nanostructured block-gradient zwitterionic terpolymer PICs via complete zwitterionic copolymerization starting from photoswitched incomplete first polymerization, i.e., AB(BC)-mode PIESA. This sequence-controlled PISA method provides the unprecedented control of the low-dimensional polyelectrolyte complex nanostructure involving not only shape but also size and thickness of micrometer-sized ultrathin PIC vesicles and lamellae, without necessarily changing the whole chemical composition and degree of polymerization.