The rapid growth of wearable electronic devices and Internet of Things (IoT) applications creates an urgent need for sustainable, flexible, and self-powered energy sources that can operate without batteries. Hydrogel-based triboelectric nanogenerators (H-TENGs) represent a new and revolutionary class of energy harvesters that address this need. These devices utilize the ionic conductivity, mechanical compliance, biocompatibility, and skin-like properties of hydrogels to harvest biomechanical energy while providing real-time, battery-free sensing capabilities. This review provides a comprehensive examination of H-TENGs, covering their composition, operating principles, manufacturing approaches, performance metrics, and multidisciplinary applications. To achieve this, this review provides an overview of advanced materials such as dual-network structures, MXene nanocomposites, self-healing materials, zwitterionic materials, and antifreezing organohydrogels. Reported power density values range from less than 1 to 10,000 mW m−2. Additionally, it critically evaluates the key challenges facing H-TENGs, including dehydration, mechanical fatigue, integration complexity, and sustainability concerns. Finally, this review proposes a roadmap for H-TENG development from 2035 to 2040, identifying enabling technologies for noninvasive health monitoring, adaptable soft robotics, deployable environmental sensing networks, and washable smart textiles.