Various examples of well-defined zwitterionic diblock copolymers have been reported in the literature. However, synthetic routes to such copolymers have almost invariably involved protecting group chemistry and/or multi-step syntheses. Herein we use reversible addition–fragmentation chain transfer (RAFT) polymerization to develop an atom-efficient, wholly aqueous one-pot synthesis of zwitterionic diblock copolymers comprising anionic methacrylic acid (MAA) and cationic 2-(dimethylamino)ethyl methacrylate (DMA) repeat units. Empirically, we find that polymerizing DMA first leads to a more well-defined block architecture and a narrower molecular weight distribution as judged by ¹H NMR spectroscopy and gel permeation chromatography, respectively. Aqueous electrophoresis studies indicate that the isoelectric point (IEP) exhibited by such zwitterionic diblock copolymers in aqueous solution can be tuned by varying the relative proportions of the anionic and cationic comonomers. The convenient removal of trithiocarbonate-based RAFT end-groups can be achieved using aqueous hydrazine, with subsequent macroscopic precipitation of the crude zwitterionic diblock copolymer at its IEP facilitating a highly convenient wholly aqueous work-up. This augurs well for potential applications of these fascinating materials. In this context, we show that such zwitterionic diblock copolymers serve as highly effective dispersants for nano-sized transparent yellow iron oxide nanoparticles, a notoriously problematic aqueous pigment.