In this paper, we provide a thorough electrochemical study of redox-active nanosized cationic gels which are promising materials for redox flow battery electrolytes. We use two-step synthesis under mild aqueous conditions: precipitation polymerization of nanogels based on poly-(N-isporopylacrylamide-co-N-(3-aminopropyl)methacrylamide hydrochloride) (PNIPAM-co-APMA), and grafting of redox-active 4-(3-carboxypropanamido)-TEMPO units to the nanogels. We demonstrate stable reduction–oxidation behavior of such nanogels and suggest a universal approach to evaluate the “effective” concentration and diffusion coefficient of redox-active groups grafted to nanogel particles. For the TEMPO-grafted PNIPAM-co-APMA nanogels we find the “effective” concentration of TEMPO-groups to be approximately 50 % of their total concentration and demonstrate an increase of the “effective” concentration upon electrode rotation. Also, we investigate electron transfer kinetics of redox-active nanogels and provide an evidence that the adsorbed layer of nanogels facilitates electron transfer.