Zwitterionic poly(cysteine methacrylate) was synthesized via reversible addition-fragmentation chain transfer polymerization (RAFT) in an aqueous medium. Firstly, cysteine methacrylate was prepared by the reaction between cysteine and 3-(acryloyloxy)-2-hydroxypropyl methacrylate in an aqueous medium via thiol-Michael reaction. The synthesized monomer was characterized by 1 H and 13 C NMR spectroscopy. Further, the synthesized monomer was used for the preparation of poly(cysteine methacrylate) in the presence of a RAFT agent in an aqueous medium at 70°C. The synthesized poly(cysteine methacrylate) was characterized by 1 H NMR and UV–Vis spectroscopy. The synthesized poly(cysteine methacrylate) exhibited zwitterionic behavior in the pH range 4.0 to 6.9 and showed an upper critical solution temperature at 44°C. Further, the antibacterial assay of the polymer was investigated under acidic and neutral conditions using Gram-negative ( E. coli ) and Gram-positive bacteria ( R. erythropolis ). Poly(cysteine methacrylate) displayed significant antibacterial efficacy under acidic conditions due to the cationic nature of the polymer as compared to the neutral medium. The minimum inhibitory concentrations (MICs) were found to be 70 and 100 µg/mL against E. coli and R. erythropolis .