Recently, one of the most important scientific issues for a better future life for humanity is achieving the ability to intelligently harvest, store, and utilize energy with good efficiency because we are facing critical environmental pollution and exhaustion of energy resources. Moreover, dramatic developments in advanced electronics, such as portable and wearable devices and electric vehicles, are being realized. Therefore, energy storage systems (ESSs) are very important for the operation of these advanced electronics. Among the energy storage materials, redox-active polymers are very attractive for ESSs because they have outstanding advantages compared with metal-based energy storage materials. For this reason, redox-active polymers are currently attracting much attention. In this review, we classify the redox-active organic groups of redox-active polymers. In addition, the latest advances in redox-active polymers and their promising application potential for ESSs are discussed with a specific focus on precise molecular designs, nanoarchitectonics, and other new approaches. Developing novel energy storage systems (ESSs) with high performance suitable for next-generation smart electronics and electric vehicles is one of the most important research issues. In addition, ESSs are also important for a better future for humanity, which is facing critical issues such as energy, the environment, climate, the food supply, and disease. We can certainly conclude that the active materials are the key parts in ESS technologies for achieving superior electrochemical properties because their energy storage capability, cycle life, and charging rate predominantly depend on the active materials. Among active materials for ESSs, redox-active polymers have advantages in terms of cost-effectiveness, diversity, good processability, unique electrochemical properties, and precise tuning for ESSs. In addition, redox-active polymers can be easily combined and hybridized with other technologies and materials. Therefore, redox-active polymers are attractive as promising materials for future ESSs. Here, we present the latest research to highlight ESS applications based on redox-active polymers with a special focus on their precise molecular designs, nanoarchitectonics, and other newly developed approaches. Based on a specific focus on precise molecular designs, nanoarchitectonics, and new approaches, we review the latest research of redox-active polymers to highlight energy storage system (ESS) applications. It is certain that ESSs are very important for future advanced electronics such as portable/wearable devices and electric vehicles. Redox-active polymers have advantages of organics, material design, and superior electrochemical properties. This review summarizes and introduces the most recent research progress and discusses the future redox-active polymers for ESSs.