Rapid measurement of the molecular weight of a polymer is a crucial task both scientifically and industrially. Usually, the molecular weight of a polymer has been determined using the gel permeation chromatography (GPC), viscosity method (Ostwald, and Ubbelohde), nuclear magnetic resonance (NMR) spectroscopy, and other methods. However, they all share a critical weakness in that it is not possible to perform rapid measurement because of the long analysis times. To overcome this drawback, we propose a novel method for estimating polymer molecular weight using electrochemistry. In this study, poly(methyl methacrylate) formed by reversible addition-fragmentation chain transfer (RAFT) polymerization was considered as a model system. As polymerization progressed, a gradual decrease in steady-state current was observed in cyclic voltammetry. The decrease in current was compared to the molecular weights of the polymer obtained using the conventional methods of GPC and NMR. Therefore, the polymer molecular weight can be determined accurately and rapidly by measuring the current.