<p>Solid electrolyte membranes based on polymers have shown promise owing to their high-energy demand and the sustainable and cost-effective nature of these materials. However, polymer electrolyte membranes composed of a polymer matrix have not progressed for the following reasons: (1) the low ion conductivity of polymer materials cannot achieve the level required for practical use, and (2) it is difficult to satisfy both battery performance and membrane durability simultaneously because of the trade-offs between ion conductivity and membrane stability. In recent years, research on composite electrolyte membranes composed of polymer nanofibers and a polymer matrix has attracted significant interest because of their improved ion conductivity, excellent membrane durability, and ability to fabricate thinner membranes. Polymeric nanofiber-containing polymer electrolyte membranes are expected to be applied not only to electrolyte membranes for fuel cells and water electrolysis, including alkaline-type electrolyte membranes for water electrolysis, but also to all-solid-state Li-ion batteries and all-solid-state Li-air batteries. This focus review presents the latest information on these topics.</p>