Geomembrane systems have been used at hydropower plants as rehabilitation and mitigation technology for several decades and are now used worldwide. In the context of planning an energy transition that promotes the sustainable use of water resources for energy production, hydropower is poised to have a major role in the upcoming decades. Geomembrane systems contribute to the maintenance, rehabilitation, and expansion of hydropower plants. They act as an impervious layer to prevent and mitigate water leakage and damage to structures. They meet the needs of many challenges faced by aging structures by improving their performance and lifespan, enhancing their resilience, and sustainability. The first application of geomembrane systems was in hydraulic canals and reservoirs and was later extended to dams and hydraulic tunnels. More recently, geomembrane systems have been applied to pressure waterways and surge shafts. This review presents a detailed overview of geomembrane systems used in hydropower and contributes to the development and share of knowledge related to their use as rehabilitation and mitigation technology. The fundamental concepts of the materials, historical development, durability, loads applied, state-of-the-art design and failure potential of geomembrane systems are presented. Subsequently, this review provides an in-depth description of exposed geomembrane systems and their application in pressure waterways as this is the most recent application type and is a field with many potential future applications. The main challenges and opportunities are emphasized, and the main research gaps are highlighted.