Poly(ionic liquid)s with non-branched, di-branched, and tri-branched side chains were synthesised using cationic glycidyl triazolyl polymers (GTPs). Based on the analyses of these cationic GTPs, the effects of side-chain branching on physical properties, such as glass transition temperature and ionic conductivity, were discussed. It was confirmed that the ionic conductivity increased with the degree of side-chain branching. Despite being a high-molecular-weight polymer (1.9 × 10⁶ Da), GTP with tri-branched side chains exhibited a high ionic conductivity of 3.6 × 10⁻⁵ M at 25 °C under dry conditions, which is higher than the benchmark value of the conventional poly(ionic liquid)s proposed by Shaplov, Marcilla, and Mecerreyes (3.0 × 10⁻⁵ M at 25 °C, dry conditions, molecular weight >10⁵ Da). From the analysis based on the electrode polarisation model, the higher ionic conductivity of cationic GTPs with branched side chains can be attributed to their higher conducting ion mobility in the polymer matrix. These results indicate that side-chain branching is effective in breaking the conventional limit of ionic conductivity of poly(ionic liquid)s.